Hi Gaute,
Before you start, make a backup copy of your whole Mach3 folder.
Install the earlier version of Mach over the top of the existing installation (no need to uninstall anything).
Your license file will still be there in the new installation.
Copy your profile .xml file (typically Mach3Mill.xml) from your backup to the new installation.
(If you have modified the 1024.set screen then you will need to copy your modified version to the new installation - same with any macro's, brains, plug-ins, etc.).
Tweakie.