how do i update the iphone 5s ?

Settings, general, software update. If there is an iOS update available, it will be shown there. If your iPhone 5s is not running iOS 12 yet, the only option you will see is iOS 12.4.1. If it says “up to date” then there is no available update for your device’s hardware.

Update to what?

Update from what?

Settings->General->About->Model,

Settings->General->About->Version

Post those 2 things and we can provide useful advice.