iPhone 6s+ Won’t Update to iOS15

Hello! I recently bought a used iPhone and am struggling to sync it with my old phone. Long story short, I figured out I need to update the phone to the newest software- currently, it’s running iOS13.3.1. When I try to install iOS15, it’ll start as normal (but very slowly) and then, several hours later, it reaches the end of the loading bar and again offers to “download and install”. Sometimes I’ll also get an error message that say *“Software Update Failed, failed to install iOS 15.0.1”. -_-

Here’s what I’ve tried so far:

• resetting the phone
• restarting the phone
• resetting network settings
• using a different wifi connection
• restoring from iTunes backup (my computer is too old to get the newest iTunes so it’s not able to download iOS15
• leaving it overnight with auto updates on

Anyone know what I’m missing here?