Right click on your worksheet tab, select View Code and paste the following
into the code module

Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
If Target = Cells(1, 1) Then _
Cells(1, 2).Value = Cells(1, 2).Value + Target.Value
Application.EnableEvents = True
End Sub

"Ray H." wrote:

What is the syntax used to update a cell value (number) by adding a value
from another cell. For example, I want cell B1 to return the value of B1
plus the inputed value of A1. In other words, I want B1 to continue to
incease by the amount that I put into A1. I am using Excel 2003.