Will my files be erased?
If I update my MacBook Pro running macOS 10.6 to El Capitan will my files get erased? There are some important files that I can't loose.
MacBook Pro
They shouldn't be erased during the course of an update and I have never had any erased. On the other hand, I wouldn't go for one minute without having both an onsite and offsite backup, whether or not I was doing an update.
Note that one of the recommended steps in the following is to make a backup: How to upgrade to macOS Catalina - Apple Support