How do I update my iOS on phone

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. 

Go to Settings > General > Software Update.

Make sure you have a good WiFi connect, that you are not using a VPN, you have at least 6 GB of free space, you have a 50% battery charge (preferably connected to power) and Low Power mode is turned off.