What do I need to do prior to update

The best thing you can & should do before upgrading is to back up all your data to an external drive. Then check the backup to make sure it's good. Then unmount (aka eject) & disconnect your backup drive before you upgrade your Mac.

Depends how much Fere Space you have on your Hard Drive & what OS you're moving to...

Caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/

If you are asking about "cleaning" apps, stay away from them. They cause more problems than they solve.

Other than removing any apps you no longer use and/or data files you no longer need, about the best thing I'd recommend is a full shutdown & restart before doing your upgrade.