How to update Value from other FORM into existing Field in another FORM ?

here's a situation.

Form A

field quantity

Form B

field quantity

How do I make that each time I entered a value in Form B ,It will update those value in the existing field in Form A

such that the

existing value in FORM A is 1 then I added value 1 in FORM B and it will update the value in FORM A into 2.

Really Appreciate it if any1 can help me..