Hello, welcome to CF.
I’m afraid the only way to upgrade an old no-longer-supported phone is to give more money to Apple. I believe iPhone 6 is stuck on iOS 12 now.
The 6 was released over 6 years ago so it had a relatively good run in terms of Apple lifespans. They do seem to try and keep the older stuff going as long as possible, at least.
A quick scan of eBay shows they still attract a fairly good price for the age so that might be one option, or look out for trade in sales so at least an upgrade won’t cost as much.
Alternatively you can try and keep using it albeit with less and less apps working. It might also be a security risk to use such an old device.