Iphone crash

Do you have a backup? If not, this is the right time to create an iTunes or iCloud backup—>How to back up your iPhone, iPad, and iPod touch - Apple Support.

After backing up restore the phone to factory settings as described in this Apple Support article—>Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If that doesn’t fix the problem most likely your phone has a hardware problem.

Most likely your storage is full. You need to delete stuff so you have at least 1 GB free.

Go to settings/General/about. How much storage does it show is available?

Lots of users have updated their 6S with no problem.

It’s not really possible to diagnose it remotely. If it has 8 GB free that’s a bit of a surprise, because iOS takes about 2 GB. So you have only 4 GB of data?

If you still have the problem after restoring to factory settings and before you restore your backup it is a hardware problem.Possibly by coincidence.