I would at as soon as possible before the paint can fully cure. I would first try a plastic scraper. If it tends to pull paint off the riser I would cut a shallow line with a box cutter between the tread and riser.

I would save heat as a last resort as you can't focus the heat only onto the paint that crept under your tape. Most of the heat will go to the tread and riser and risk loosening or bubbling them.

In the future... after putting down painters tape seal the edge well. I use a putty knife or piece of metal and firmly slide it along the edge of the tape where I will be painting to get that edge stuck down well. I don't touch the rest of the tape so it's easier to remove.

 

Next time try Frog Tape.
Does a much better job sealing the edges than blue painters tape.

 

Regardless of the tape, if you lather on large volumes of paint there is a chance it's going to leak.

A good technique to use it to lightly paint the edges with a relatively dry brush, that paint will form a seal from the tape to the painted surface.

When that dries put down your coat(s) of paint and you will have much less bleed through.

Use this and you can actually save a lot of $ by not buying the expensive tapes!

 

Often the best way to remove dried paint like that is to carefully scrape it off with the edge of a sharp knife.

 

I saw frog tape at the paint store but I went on the cheap. I also was in a rush. Thanks