How to update my iPhone to iOS 13?

If you want to update iOS then you need a newer iPhone model.

Your iPhone 6 cannot update past iOS 12.5.1.

You have an iPhone 6, Tejas. An iPhone 6 can ONLY be updated up to iOS version 12.5.1 and you already have that version. You will need a newer iPhone to update to iOS version 14.4 (current version).

Axel F.