Is this a federal loan? Only federal loans can be rehabilitated.
Is the loan in default? (Has it been assigned to a collections agency?) Only defaulted loans are eligible for rehab.
If you only have a single late, your only option is to request a goodwill adjustment of your credit report from the lender. If you read some threads here, you can learn about other people's experiences and the tactics they used. Results are mixed, but with persistence, it does sometimes work. You should also note that a single 30-day late from a year ago is already losing its sting on your score. If you keep paying all your accounts on time, it will really only have a minimal impact after another year or so.