Understand Plex licensing is troublesome for the Amahi team but you should probably remove Plex unless it can be fully supported with updates.
We do not intend to remove it from Amahi! If you choose to use the app, that's great. If you want to update the app manually, that's your choice but understand you do so at your own risk.
Best way to keep it updated is by submitting a bug report
for new versions. Keep in mind the update may take a few days, depending on our workload.
Understand we have a few hundred apps to maintain, so trying to keep them all current is a herculean task. We try to stay on top of updates, but often it's difficult with such a small team. Also we only update apps for the latest Amahi release, which is Amahi 11 on Fedora 27.
If you'd like to help keep it updated, we are always looking for users who can lend a hand. It does require some advanced linux skills, but can be easily taught if you are interested.