Use Edit|Undo to undo the last editing action. Repeat to undo more actions.
You can undo the undo with Edit|Redo if you do so right away. If you take any other action that is remembered by the undo function, the redo list will be cleared. Only trivial actions such as moving the cursor do not clear the redo list.
You can undo several actions in one go via the submenu of the Undo command. When you select an action, that action and all actions listed above it in the submenu will be undone at once. They will all be added to the redo list too.
If you use the menu item or toolbar button to invoke this command, it will be invoked on EditPad's main editor, where you edit files. If you press the shortcut key on the keyboard, it will be invoked on whichever editor is showing the text cursor (vertical blinking bar), whether that's the main editor, the search box, or the replace box.
The undo feature remembers all changes you made to any file since you opened it in EditPad. Saving a file does not clear the undo list. Switching between files switches between the undo lists of those files. EditPad remembers the changes you made to each file even while you work with other files, until you close each file.
Undoing and redoing actions updates the file's "modified" status as indicated by the status bar and tab color. When you undo all changes you made since last saving the file, or redo all changes you undid since last saving the file, the file will be indicated as being unmodified.
EditPad may use quite a lot of memory to remember all the changes you made to all the files you have open. Some commands, such as a search-and-replace across all files, may result in very large numbers of changes. EditPad's undo history includes a safeguard to make sure EditPad does not run out of memory. If the undo history grows too large, EditPad automatically discards the oldest changes. In extreme cases, like a search-and-replace that makes millions of replacements, the undo history may be cleared entirely.
In EditPad Lite, the memory limits are set automatically to make sure EditPad Lite doesn't use up all of your computer's memory, while still being able to keep a complete undo history. Unless you're making millions of changes in a search-and-replace, EditPad Lite's undo history can easily keep track of a full day's worth of text editing.
In EditPad Pro, you can configure the limits in the System Preferences. The default limits use the same balance as EditPad Lite. The actual numbers depend on the amount of RAM your PC has.
In practical terms, you needn't worry about EditPad's undo history. It'll remember virtually everything, without causing your computer to run out of memory and crash. If you do need to undo very long editing sessions, EditPad's backup options and File History may be more practical anyway.