Refinance the loan in your name only. If your co-signer wants to stay on the loan but you don't want them to be the primary owner of the car anymore, you can refinance the loan in your name only. This will require that you qualify for a loan on your own, without the co-signer's help. However, it will also give you full ownership of the car and you won't have to worry about your co-signer's credit history or financial situation affecting your car ownership.
Sell the car and buy a new one in your name only. If you don't want to refinance the loan and you can't come to an agreement with your co-signer about how to transfer ownership of the car, you can always sell the car and buy a new one in your name only. This will obviously be the most expensive option, but it may be worth it to you for the peace of mind of knowing that you're the sole owner of your car.