No, it is not possible to reduce the mileage on your contract mid-contract. It is possible to increase it where required and usually works out cheaper during the contract than paying the excess mileage charge.
Mileage is worked out over the duration of the contract, so should your contract include 1,000 miles per month but you do 500 miles in month 1 and 1500 miles in month 2, this will balance itself out.
It is extremely important that you estimate your mileage usage correctly before taking on a contract, so that you don’t get any nasty surprises at the end.