I have iso 12.5 but need to update to atleast iso13 for apps and I don’t know how when I hit updates nothing is showing up how do I update to iso13

You can't. An iPhone 6s won't be running 12.5; they went from 12.4.1 to 13.0.

An iPhone 5s or 6 doesn't have enough RAM to run 13 or above.

(188819)

Go to Settings > General > About > Model Name and verify the model. It sounds like you may have an iPhone 6 rather than a 6S. The 6 cannot be updated beyond iOS 12.5

If it is a 6s it should update to the current iOS 14.3 via Settings > General > Software Update.