You don’t. That is why most people take the SAT in 11th. Now assuming you are in 12th, and the next SAT date is October you have around 2 months to prepare. Take a diagnostic test, analyze your mistakes. Buy some books (look them up or leave a comment) and study for around 1 hour a day along with NEET. Do more on weekends.
This is the only way out, you can’t take the December test since it’ll be too close to boards. Keep in mind that if you want to do a pre medical course you’ll have to take the subject tests in November.