Return to MuseScore2

If you mean 112% of the tempo in the Play panel, click the number with the % in it and type a new number. If you mean the zoom on the display then you can do the same thing there.

You can have Museises 2 installed together with MuseAScore 3 and upgrading to 3 won't have uninstalled 2
If you manually insiatelled it, you can get it from https://musescore.org/en/download#Older-and-unsupported-versions

But your tempo problem is certainly unrelated to this, you can set any tempo you want

It sounds like maybe you have been making the mistake of using the Play Panel as a way of setting the tempo for your score but that has never been its purpose, it is for temporary overrides to the basic tempo. The basic tempo is controlled using tempo text from the palette, which allows precise control in beats per minute and more importantly, will also be seen by the musicians reading your score whereas the Play Panel obviously will not.

That said, the play panel continues to allow you to enter whatever percentage you want as an override, not sure what is preventing you from doing so but just type whatever you want. Or, if you prefer dragging the slider, simply undock the play panel and make it bigger so the slider will give finer control. But again, that is not and never has been the correct way to set a tempo.

For the time beeing v3 is a big disappointment ! I seriously think about revert to v2 :-(