You can use:

Memo .Perform(EM_UNDO, 0, 0);
If you want to check whether undo is available, so you can enable or disable a menu item choice, you can check the "Undo status" like this:
If Memo .Perform(EM_CANUNDO, 0, 0) <> 0 then begin {Undo is possible}
end;
To preform a "Redo" simply "Undo" a second time.