Double click on the MAMP_MAMP_PRO_6.xx.pkg file in your Downloads folder to start the installation process.
The installer will rename your existing /Applications/MAMP to /Applications/MAMP_current_date.
This folder can later be deleted. You may however keep it, so you can fall back to your original setup.
Your existing htdocs folder will be moved to your new /Applications/MAMP folder.
Confirm that all data has been transferred correctly before starting MAMP PRO.
Your /Applications/MAMP_current_date folder can now be deleted. You may however keep it, so you can fall back to your original setup.