Q: Last time I updated my iOS (to El Capitan) it completely wiped my computer. Will that happen again if I upgrade to Sierra?

It didn't wipe my Macbook Air, upgrading from El Capitan to Sierra. Also, just a note, iOS is the mobile operating system for iPhone and iPad, OS X is for Mac.

If you use Time Machine application to create a backup of your system

that can be used when you upgrade a computer to restore most of former

content into a later macOS. There are some rules, methods & suggestions.

Easier if you have a Time Capsule device to use with Time Machine; or

if you don't, an externally enclosed hard drive can be used for this task.

Usually the new system installation does not overwrite the user files; but

if you used the OS X Recovery and disk utility there, or made choices to

allow the new installer to erase & install, perhaps older content gets lost.

Backup, also keep a secondary backup; beyond Time Machine's saved files.

Good luck & happy trails!

There is no option to, nor is there any code in the installer to erase your hard drive during installation. There hasn't been that option in the installer for at lease the five previous OS's.

If you use Disk Utility prior to installing the upgrade to erase the drive, then your drive will indeed be erased.

I have seen people here create a new user during the setup assistant. They then think that all of their files were deleted because they don't see them in the new user account they logged into. The files sit happily in their old user account.

I don't know why they thought they should create a new user, nor why they would expect their files would be transferred from the old account to the new one.