How can I update my iphone6 plus ios15

An iPhone 6 plus can't be updated beyond 12.5.7, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

To update to 12.5.7 -

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

What isn't working on your phone?

But this will not work, it is end of the line for this iPhone and it will never update further then iOS 12

If you need newer software you will need a newer iPhone

You cannot update to iOS 15, your phone doesn't have the hardware needed to run a higher version of the operating system.

You still haven't told us what isn't working on your phone.