I need to update my iPhone to 13.5

An iPhone 8 or newer is required for and can update to the current iOS 16.

An iPhone 6s or iPhone 7 can get to iOS 15.

iPhone 6 and earlier are limited to iOS 12, at most.

If you want or need an app that requires iOS 13.5 or later, you will need an iPhone 6s or newer.

Here is how to upgrade iOS: How to Upgrade Your iPhone - Apple

There are no iPhone hardware upgrades available. Only a newer (used, refurb) or new iPhone.