How to update 'Money out' starting balance based on the selected month in a Budget Planner

Hello I already have a prototype that I have done, I am doing the Budget planner for my self. So I want to keep track of my expenses which I already did in "Money out" table with pop-menu for each month. I have "Money In" table where I listed every paycheck that I will get and other income. Also paycheck has a formula where it calculates the amount. So what I want is or actually what problem I encountered with is that when I change the "Money out" to April instead of using April starting balance which is Current Balance of March it uses the Starting balance of March.

[Re-Titled by Moderator]