I can't change the tempo?

[Faexie]

I tried to change the tempo from 140 to 100 but whenever I hit play in the music editor it goes back to 140 bpm... It doesn't happen in the beat editor though

[zonkmachine]

Right click the tempo view and choose Remove song-global automation.

[musikbear]

I tried to change the tempo from 140 to 100 but whenever I hit play in the music editor it goes back to 140 bpm... It doesn't happen in the beat editor though

It is because you are importing a MIDI -right?
The MIDI has the BPM 'baked' in as a global-variable. So just as zonkmachine says, you need to delete that variable in the midi-file.
This is where you right-click:

Besides that, since you are new in Forum
Welcome Faexie !
Here are all important links:
viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions