How to download an old version of the program?

Hello @za30926654c3hg,

Thanks for the question.

If you have a Creative Cloud version, it's pretty easy:

If you are with Steam, you don't have access to as many version as with the Creative Cloud (due to Steam's restrictions). You have access to the last minor (8.3.1) of every major (8.3.1) from the year you bought. 
RMB on Substance 3D Painter > Properties > Bêtas 

I hope this help.