How to add new data to existing cell based on multiple selective inputs?

I know what I want to do, just not how to do it or describe it, so please forgive me if this doesn’t make any sense.

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. I hope.

Maybe like this with a command button instead of comboboxes.

Private Sub CommandButton1_Click()
Dim Course As Range, Level As Range, Hours As Integer, r As Long

Set Class = Range("B2")
Set Level = Range("B3")
Hours = Range("B4").Value

If IsEmpty(Range("B2")) Then
    MsgBox "Please select a Class"
    Exit Sub
ElseIf IsEmpty(Range("B3")) Then
    MsgBox "Please select a course Level"
    Exit Sub
ElseIf IsEmpty(Range("B4")) Then
    MsgBox "Please enter number of Hours earned"
    Exit Sub
Else
    r = Application.Match(Class, Columns(4))
    Select Case Level.Value
        Case "Intro"
            Range("E" & r).Value = Range("E" & r).Value + Hours
        Case "Basic"
            Range("H" & r).Value = Range("H" & r).Value + Hours
        Case "Intermediate"
            Range("K" & r).Value = Range("K" & r).Value + Hours
        Case "Advanced"
            Range("N" & r).Value = Range("N" & r).Value + Hours
        Case Else
    End Select
End If
Range("B2:B4").ClearContents
Range("B2").Select
End Sub