Undo

Used to reverse recent edit operations. You can set the size of the Undo buffer in Options/Preferences. It can store up to 512,000 edit operations. It is set by default at 1,000.

Undone entries can be redone using the Redo command.

If an edit operation such as a Merge or a large Paste exceeds the current Undo buffer size, you will be advised on completion. The buffer will be temporarily extended so that you don't lose any information. Before you do anything else, go to the Options/Preferences menu and change the Undo buffer size. The temporary buffer will continue to hold the Undo information until you try to enter or edit an entry. Ctrl/z will still Undo and Shift/Ctrl/z will still Redo. (This, incidentally, is a good way of checking whether your new buffer size is adequate - Undo the large operation, change the buffer size and then Redo it. No warning message means there is enough space.)

Use:
Ctrl+Z

Edit menu