If you have credit score less than 650, it could mean, either you have mismanaged your credit card, defaulted on your loan repayment or the least it could be an error on your credit report. To improve your score to 750, you need to first get and study your credit report. If you were paying only minimum due on your credit card, try to pay it in full to improve your score.
Similarly, never miss a repayment on your loan and pay your credit card bills on time; late payments will have a negative impact on your credit score. Lastly, never exceed on your credit limits and keep it below 50% of the limit to keep improve your score steadily.