How to update my iphone 6s, it is indicated there that ios 12 is up to date and I cant even download some of the apps.

If it says you are up to date at 12.5.5, then you have a 6 or 6 plus, not a 6S.

Go to Settings > General >About and verify your model.

The highest version of iOS that the iPhone 6 can install is iOS 12, so if you would like to update to a later version of iOS you will need to get a new phone.