From BlenderWiki

Undo and Redo

Blender has many options and features to make sure that you do not lose your work. First, it saves your actions in a list. At any time, you can tell Blender to back up in the list and undo most recent changes. Second, when you start Blender, one of the File options is to Recover Last Session. When you exit Blender, it saves the current file in a quit.blend file – Recover Last Session merely loads that file back in. Third, you can tell Blender, via the user preferences, to automatically save versions “behind the scenes”, and/or to keep old copies of your entire files every time you do manual saves.

Getting Started

By default, undo is not turned off although it takes precious memory. To enable or disable undo, drag down your User Preferences window and click Edit Methods. In that panel, you may set:

• Steps – This numeric slider sets how many steps, or actions, to save. If you set this to 32, you will be able to undo the last thirty-two actions that you performed.
• Memory Limit – This numeric field allows you to define the maximum amount of memory, in megabytes, the undo system is allowed to use. If you let it to its default 0 value, there is no limit.
• Global Undo – This enables Blender to save actions outside of some mesh editing actions, for example, moving individual vertices while a mesh is in one editing session. Each vertex move can be undone.

Undo

When you have done something terrible to your beautiful model, you have the following choices:

1. Keep working forward and try to cover up or build on your accident.
2. Undo (via CtrlZ).
3. Revert to (i.e. open) a previously saved version in your working directory.
4. Or regress to the auto-saved version (if you have Auto Save turned on).

The first CtrlZ undo you should already know – it’s a very common feature in most programs ! Just note that in Blender, there are two separated histories : one dedicated to the Edit mode, and the other for all other modes (and especially the Object one).

So let’s focus on the “file undo” features, a.k.a. “auto-save”.

Auto Save

The creative process is very involving, and the artist often gets so deep into modeling and animation that he or she often forgets to bathe, eat, and especially save copies of their work. A computer crash, power outage, or simply taking a bad fork in the creative path can result in lost work or corruption of the desired product. Have no fear of immersing yourself, because Blender provides several ways to automatically save backup copies of work in progress. The Auto Save tab of the User Preferences window allows you to configure the two ways that Blender allows you to regress to a prior file version.

Save Versions

This control tells Blender, when you manually use File » Save, to keep the specified number of previous versions of your file. In your current working directory, these files will be named .blend, .blend1, .blend2, etc., on up to the number of versions you specify, with the older files named with a higher number. Typically, nine versions are more than sufficient!

Auto Save Temp Files

Enabling this button tells Blender to automatically save a hot backup copy of your work-in-progress to the temp directory. It will reveal two more controls. The first Minutes one specifies the number of minutes between each automatic save. The auto-saved files are named using a random number, have a .blend extension, and are placed in the Temp directory (refer to the File Paths tab). We recommend that you use these buttons to auto-save into your temp folder file path, and set how many minutes go between automatic saves (five to ten minutes should be enough).

The second Open Recent button allows you to open the most recently auto-saved file. Upon loading the auto-saved version, you may File » Save it over the current file in your work directory as a normal .blend file.

There are a few points worth to be noted:

• Clicking the Open Recent button will immediately load the most recent save, and you will lose any changes that you have made in the intervening minutes.
• Only one auto-saved file exists for each project (i.e. Blender does not keep older version – hence you won’t be able to go back more than a few minutes ago, with this tool).
• The auto-saved files are deleted as soon as Blender is closed – they are really temporary working backups, not long term ones!

Recent Files

This setting controls how much recent files are listed in the File » Open Recent sub-menu.

Save Preview Images

I think this should pack in .blend files the render buffers… But it doesn’t seem to work – if you know more about it, please edit this page!

Redo

Just as CtrlZ undoes an action, ⇧ ShiftCtrlZ re-does the last undone action(s).

History

AltU displays the Global Undo History (or Editmode Undo History if you are in Edit mode…) of what you have done as a list of actions generally cleverly named. Clicking on any action reverts you back to that state just before the next action was performed.