I want to update my iphone 6 software to ios 13.5

it's 100% up to the developer of the apps which ios versions they will support

your options are

use another app that has lower requirements

or reach out to the developer and ask them to support older ios versions

or get a new iphone

iphone6 has hardware which does not support higher than Current: iOS 12.5.5, released September 23, 2021[10

iPhone 6 - Wikipedia