Most universities will let you audit a course, even graduate courses (if you can convince the professor you have experience in the area) just by paying a fee for that one class. You don't need a credential to do this.
Auditing classes is fine¹ but probably not enough value for me on its own to be worth the time and opportunity cost. I'd want the degree and I'm fairly sure I have the skills and knowledge to begin a CS masters. But I can't get one without doing a BS first, which makes it overall just not worth the effort.
¹ Actually is it? I hear this recommended a lot but I don't know anyone who has audited a university course while not being a student. I tried it once at my local state school but it was an amazingly complex process to get signed up, and then I was told I could have a slot if there was ever an open one. There never was.
How? I have quite a few colleagues with Master's in CS who are no more competent or well paid than me with no degree at all.
Oxford Software Engineering and Coursera Colorado Electrical Engineering will take people without Bachelor’s.