Petition credit by exam

I’m having a difficult time getting into psyc012 for repeat and it’s more likely that I won’t be able to get in which will result in the department to dismiss me from my major which is psychology. Since I did poorly last quarter, I’m not eligible to petition credit by exam; I want to stay in my major but how does it work after the petition is approved? Also, if I want to keep my major, what should I do?