As a private seller can you repo your car if the buyer is a day late or can you change the contract?
If the contract to sell the car was silent regarding under what conditions you can repossess the car, then you must seek relief through the courts. It also depends upon the laws in your region - some jurisdictions to not empower private repossession - in these jurisdictions all repossessions must go through the courts.
Once the judgment is awarded, it is then recommended that you obtain the service of the Sheriff's office to escort you on the premise to retrieve the property due to you. Or you may contract a repo firm.
Unless the contract specifically allows you to, you can never change a contract unilaterally. You can only change the contract if the all parties agree.