Either go with working through textbooks on your own, as has been stated by others, but that approach requires tons of discipline as the subject matter can get quite dry and frustrating at times. Or, I’d say ideally, enroll in courses at a local community college. The one I went to had four calculus classes, linear algebra and differential equations, along with statistics and I believe some kind of math programming course (my have just been part of other classes). So you cab get into fairly advanced stuff going that route.
If you want to learn even more after that, you’ll have a solid basis to continue self study. Hell, you might even have a good start towards a bachelor’s in math.