r4lf wrote:
Probably best to copy everything externally and then do a full factory restore. Then upgrade to the latest possible iOS version the device can run, which is 15.8.4.
Once it's set up as a fresh device, copy things back from external to set it up the way you want it.
Whatsapp will require an iCloud backup from inside the Whatsapp app internal settings to pick up messages when reinstalling the app, and please when recommending a full restore, recommend a backup first, or syncing all data to iCloud. Saving photos externally is not so easy with iphones without using a cloud server, and iCloud Photos would be recommended.
Support links help.
Back up iPhone – Apple Support (UK) (in case it all goes wrong)
Backup Whatsapp
Use iCloud on iPhone – Apple Support (MY)
Erase iPhone – Apple Support (UK)
Set up your iPhone or iPad – Apple Support (UK)