You need to be jailbroken in order to use it or have the iOSGods app for it so you can sideload the IPA with iGameGod installed within the app itself.
However you will need the iOSGods premium subscription in order to do that.
Seeing that you have iOS 14.7.1 and an iPhone 11 though you can just Jailbreak, install the iOSGods Repo to Cydia/Package manager of your choice and install iGameGod, then go into the iGameGod app once it finishes installing and toggle use for Beatstars.
Once you’re in Beatstars you will notice a pink cog with a wrench in it, tap and hold on the floating widget, you’ll see two other options, select ‘Speed Hack’ you should be able to speed up/slow down how fast everything in the app goes from there.
Hope this helps.