To undo your previous actions, press Ctrl + Z (for Windows)/Cmd + Z (for Mac) until you reach the action you want to undo. If you prefer mouse navigation to keyboard shortcuts, click undo.
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To redo an action you've undone, press Ctrl + Shift + Z (for Windows)/Cmd + Shift + Z (for Mac) or click redo.

Both the redo and undo buttons get activated only after at least one action on the board. You can undo/redo only your actions, not your collaborators'
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