1. Download MAMP PRO 2.0 here: http://www.mamp.info/en/downloads/index.html
2. Backup your old MAMP and MAMP PRO data (just for security reasons)
3. Open the Installer and follow the instructions.
You don´t have to remove your old version of MAMP & MAMP PRO. But as always, we would suggest to backup both.
If everything goes well, your projects and preferences will automatically be restored by the installer of MAMP PRO 2.0.