Make sure you have the official REX release 20.1.3, before you start updating.
You need to Install Naviate S and then update it to 20.1.6 first.
Then you van install Naviate REX 2020.1.3 and update it to 20.1.5
If you get this message:
You need to Install Naviate S and then update it to 20.1.6 first.
Then you can install Naviate REX 2020.1.3 and update it to 20.1.5