You’d have to contact your university to withdraw
Don’t know if you meant it to read the way it does, but it basically reads as:
How do I quit uni? I’d rather throw away all that money and get no qualification at all instead of receiving a qualification at a lower grade than I’d like
If I were you I’d think about it, generally a low grade is better than no grade at all. And discuss your possibilities with your university first, there may be aspects to it that you haven’t thought of and they may be able to provide more support or aid you in some way. I wouldn’t make a decision until you do that as it may change your decision entirely