1) Always upgrade your DEV/TEST Environment before PROD to verify your upgrade process.
2) If you are using Enterprise Tester 4.10 or below, you must upgrade to Enterprise Tester version 5.1 in the first instance, then upgrade to Enterprise Tester version 6.0. You can then upgrade to the latest version of Enterprise Tester.
3) Enterprise Tester 6.1 and above requires .NET framework 4.6.
4) Always check external system compatibility below before upgrades.