When I went to college in my mid-thirties, many of the topics covered in modern high school algebra had not been taught in high school. Based on the pre-tests, I was advised to drop out of algebra and take a remedial course instead (obviously had forgotten a lot!)
What I did was go to the college bookstore to find a self-study workbook that I went through step-by-step to prepare myself for the class. I also used it as a secondary resource for the homework assignments from class. I worked all the assigned problems, plus the problems in the relevant self-study guide, and probably put in about four times as much time on homework as my classmates.
The professor graded on a curve. As the first test was handed back, he told us the scores ranged from 32 to 98. You guessed it. I had the 98. I was not at all confident about my understanding of what I was doing; all I can say is that spending a lot of time studying worked!