Add New Data To Existing Cell Based On Multiple Selective Inputs?

I have a spreadsheet of courses required to reach a certification. On this spreadsheet I have listed the number of hours required for each course in one column, and how many hours I have accrued in an adjoining column. Not all the hours will occur at once, so I tend to bound from cell to cell adding hours in small amounts. What I am trying to do is create a macro that will allow me to add to the existing number of hours to the newly accrued hours, without typing over what is already there.

For example�Class 1 requires five hours total, and I have two hours accrued. If I accrue two more hours (for a total of four hours) I want to update cell E2 without going in to this cell manually and changing this number. I would like to enter the additional two hours in a text box or similar function, and have that function update E2. To add to the level of difficulty, there are four levels of class. This means not only do I need to be able to select which class hours need updated, but which level of class. I have attached the spreadsheet I am working with to try to make things a little clearer.