How can I have multiple tempo changes within one song? GB for iPhone

Hello all! So I finally picked up an interface and have started the journey of demoing/ pre proing out my own music. It's been going well and all up until I needed to make a tempo change around the middle of the track. I figured first drop the bpm from 155 - 130 then track the instruments. after all the guitars and drums were tracked at 130 for that part I rose it back to 155 but as soon as I did bars of my song started shifting on their own and scrambling up. Its incredibly frustrating when you take a lot of time to track a song for it to go up in smoke :/ is there anyway I can pull off multiple tempo changes in one song? All my songs have "drops" at some point and if I can't have multiple tempo changes on a track than that makes GB for iPhone pretty much useless to me 😟 any insight is very appreciated!