The basic undo command in Emacs is:

  • C-/ (hold Control and press /)

There are also equivalent keybindings you can use:

  • C-_ (Control + underscore)
  • C-x u (Control + x, then u)

All three run the same undo command; C-x u is often easier to remember, while C-/ or C-_ are quicker once they’re in your muscle memory.