An iPhone 5s will update as far as iOS 12.5.7 Any equipment currently running iOS 12.5.7 can't be updated to anything higher.
This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support Read the article for recommended criteria for being able to use wireless updating.
You can also use Mac and PC computers to update system software --> Update iOS on iPhone - Apple Support
If you want a video about doing this, follow this link —> https://www.youtube.com/watch/1FcE86kwODQ
When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date (and can't go any higher) and won't install anything. If you want to run a higher system version than your current device is capable of running, you need to buy a newer device.