How to update my iPhone 5s?

An iPhone 5S can't be updated beyond 12.5.5, 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.

If you need to update to 12.5.5 -

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone, iPad, or iPod touch - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support

On your iPhone.......

Home Screen > Settings > General > Software Update

Tell us what information appears on your iPhone's screen