Oppurtunity form Estimated revenue need not to be fixed

Estimated revenue need not to be fixed how to do this.. i will explain in detail1. this is 1st oppurtunity field here when i go and click april estimated ammount    

2.its showing 2345.003 if i go to second oppurtunity when i click april estimated ammount i need different ammount but i am getting same 2345.00 how to achieve this