How can I get OS 15

An iPhone 6 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 Update your iPhone or iPad - Apple Support

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

Update iOS on iPhone - Apple Support Update iOS on iPhone - Apple Support

Then you will need to purchase a newer phone capable of running a higher iOS