If you installed it manually, via the makefile - then use the uninstall target of the makefile to uninstall it.
If you installed as root, you will need to su to root, or use sudo in order to run the uninstall command below.
NOTE: If the original installation did not complete successfully - you'll almost certainly get errors from the uninstall script too - which is to be expected.
After running the uninstall script - you can then remove all of the python 3.8.5 related stuff from /opt/.
If that doesn't work, then you'll have to remove things completely manually.
So you'd need to take a look at the makefile and read through the commands used in the altinstall build-target and see what things were copied where.
Then you'll have to manually remove those items.
But do so very carefully, or you might bork something!