The 'u'ndo command is used to undo or redo write changes done on the file.
> u?
Usage: > u 3 ; undo write change at index 3
> u -3 ; redo write change at index 3
> u ; list all write changes
Here's a sample session working with undo writes:
[0x00000000]> wx 90 90 90 @ 0x100
[0x00000100]> u ; list changes
00 + 3 00000100: 89 90 c4 => 90 90 90
[0x00000000]> p8 3 @ 0x100
90 90 90
[0x00000000]> u 0
[0x00000000]> p8 3 @ 0x100
89 90 c4
[0x00000000]> u -0
[0x00000000]> p8 3 @ 0x100
90 90 90
Note: Read 'undo-seek' for seeking history manipulation.