UPGRADE FROM 3.X TO 4.X
It is recommended to make a copy of your site, for example at staging.yoursite.com to test upgrading from 3.x to 4.x.
At the very least you should have a backup that you can revert to in case of problems. We recommend Akeeba Backup.
UPDATE TO THE LATEST VERSION
MyMuse uses Live Update from Akeebabackup. To get the new version, you must go to Components->MyMuse and click on Options at the top right. Enter your username and password from this site into the tab "Live Updates".
Go to Components->MyMuse and you will see a notice when there is a new version. Click on
If you see the error "Unable to open archive
Extracting the update package failed. Please try updating the extension manually". Please click on "Refresh Update Information" and try again.
The software will be updated.
You can also come here and download the latest version. Install the latest version. It will detect that MyMuse is already installed and not touch the database, simply overwriting old files with the new ones.
IMPORTANT: open Components->MyMuse->Store and save once to establish new options.
MyMuse will save the css file as components/com_mymuse/assets/css/mymuse_old.css
If you have made changes there please compare the old and new to bring in your changes.