*** READ BEFORE UPGRADING***
ANY CUSTOM CHANGES YOU HAVE MADE TO THE THEME'S TEMPLATES WILL BE LOST UPON UPGRADING. YOU SHOULD BACKUP ALL CUSTOM ADDED CODE AND REINSERT AFTER RUNNING THE UPGRADE.
1. Upload the files of your new theme package similar to step 1 of Installation. Select Overwrite - Yes to all files.
2. Browse to the .xml file similar to step 4 of Installation
3. However, change the option to 'Overwrite Style' and select your existing parent theme to overwrite.
4. Click 'Import'. Your theme will now be overwritten/upgraded.