I want to update my phone

Go to Settings/General/About and see what the model of your phone is. If it is a 6 or 6 plus you cannot update it higher than 12.5.5. If it is a 6S or 6S plus see→Update your iPhone, iPad, or iPod touch - Apple Support or watch this video:

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Are you absolutely sure you have an iPhone 6s Plus, and not a 6 Plus? The latter cannot be upgraded past iOS 12.x.