I un-installed Avast, but I keep getting a pesky pop up that wants me to upgrade .
A screenshot would help to explain what popup you are receiving.
Did you run the Avast clean tool to remove leftovers after uninstalling Avast? if not please do so.
It means that avast wasn’t removed completely.
Remove all leftovers (folders, files, registry keys) manually.
Avast removal tool:
https://www.avast.com/uninstall-utility