I want to update my iOS version

The link on top of this page which you had visited, before posting this question here, is shown under the "Related Article” that may have given you an answer. 

You need to mention which part from the above article was not clear to you.

You are asking for an update (higher iOS versions probably) because there are a few apps that do not work on iOS 12.5.6. You should contact the app developer for the same.

Ask the App developer to release the app for lower iOS versions also. Netflix has done it.