You should be able to use a normal calculator if it allows you to enter the term. Trulia offers one at
http://www.trulia.com/mortgage/calculator/payment/ . I you want help working through it, feel free to contact me. I'm in Decatur, and would be happy to go through the details. There are some factors that could be addressed in your specific loan that will affect the actual payment, but in a straight forward scenario, just use the calculator referenced above adjusting the months to the remaining term of your loan and the new interest rate.