How do you change the font in Substance 3D painter

If you mean adding a custom font then you can do that by creating an sbsar file in Substance Designer.

At it's simplest it is a text node connected to an output node. However the font type cannot be exposed as a parameter so here I have made a node using Franklin Gothic Book font regular and a second using Franklin Gothic Book font italic. I could have added more. The outputs are connected to a multi switch node which is driven by an exposed dropdown list. So selecting "regular"in the dropdown list uses the first node, selecting "italic" uses the second node.

For one of the nodes I exposed the font size and text string parameters, then I set the second node to use those same inputs.

Finally publish as an sbsar file into your shelf under "Alphas".

Dave