I want to upgrade my iPhone

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

Tap Settings > General > Software Update

If the message says that your phone is up to date, there are no more updates available for your old iPhone 6 Plus phone.

If an update is available, it will appear and you can tap to install.