Since there is no way to select the levels, the only solution is to start again, here's what to do to select a previous/following act:
Find the file "gamelet.save" in Windows (for WinXP "\Documents and Settings\[user name]\Application Data\mif2000's Hamlet\").
Open the file in Wordpad.
Change "actual act" to "3_0" for example to go to (Act 3)
Save the file and run the game.
If you find any difficulty using the help system in the game, check this walkthrough includes tips and tricks, helpful hints, and a strategy guide to complete Hamlet.
http://www.gamezebo.com/download-games/hamlet/walkthrough This is another point and click adventure game with nice graphic and sound which is very similar to Samorost as a whole it is worth playing it.