I've downloaded the package 3x and try to install it but still no joy. I think the issue come from the invalid digital signature/certificate but how to fix it? Thanks!

It means there are still some files from the old Facebook in your phone or you just already have the same app installed. 1) Reinstall the orignal Facebook/Messenger 2) Go in settings, then Applications manager 3) Go until your original Facebook/Messenger

Android App Not Install. An existing package by the same name with a conflicting signature is already installed. I'm still in the testing phase of this upgrade, so the file I download is a signed apk of a previous version, which I think should work wi