First stop the Server if its running. Go to "/Applications/MAMP/bin/", rename the PHP Version you don't need (MAMP is only allowed to use 2 PHP Versions), e.g. "_php5.2.17". Now MAMP will use the php versions that are left. Go to the MAMP Manager and then settings, then switch to the php version you need.
One problem with this solution I encountered was the httpd process (took me a while to figure that out xD). If you have the httpd process running in the background, then the php switch won't work, until you stop those processes (sometimes MAMP has an awkward problem to stop the server, thats why this process can be still alive). Start your Activity Monitor on your Mac (Shortcut: Press Command+Space and type in activity...), go to the Search Function and type in "httpd", close all those processes. Now you should be able to switch your PHP Version with the MAMP Manager.