How to update a theme that is NOT a wpmu theme. We are using Divi

Our site is a multisite and we are using Divi for the network theme. Each multisite is using wpmu themes. The Divi theme will not allow us to update. It is currently out of date (1.3 version) and needs to be updated to the latest 2.0 version. Can you please help us update the Divi theme. I'm concerned the only way to update is to delete it entirely and reinstall. Please help!