How do I update my OSX (El Capitan)

[trs96]

By changing the disk format I'm worried data will be wiped from the Mac SSD. Is this something to worry about?

You should always make a bootable backup first. When you update, Clover will not be affected because it's on your EFI partition, not on the OS X El Capitan partion. It shouldn't get overwritten when you update to Mojave.

 

[phoenixAL]

You should always make a bootable backup first. When you update, Clover will not be affected because it's on your EFI partition, not on the OS X El Capitan partion. It shouldn't get overwritten when you update to Mojave.

Definitely making a bootable and a time machine backup.
It's been said that updating from 10.11 to 10.15 is not straightforward (www.tonymacx86.com/threads/big-update-any-help-is-appreciated.276822) and they suggest to do a clean install.