OK, I have an idea of what you want, but without any idea of your sheet's layout, or how you determine which dates get updated or by how much, here's a fairly simple approach.
You can select whichever cell(s) you want to change, run this code and input how many days you'd like to add to the selected dates.
<font face=Courier New><SPAN style="color:#00007F">Sub</SPAN> ExtendDueDate()
<SPAN style="color:#00007F">Dim</SPAN> i <SPAN style="color:#00007F">As</SPAN> Range, x <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">Integer</SPAN>
x = <SPAN style="color:#00007F">In</SPAN>putBox("How many days do you want to add to these dates?", "Due date extension")
<SPAN style="color:#00007F">For</SPAN> <SPAN style="color:#00007F">Each</SPAN> i In Selection
i.Value = i.Value + x
<SPAN style="color:#00007F">Next</SPAN> i
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN></FONT>
Hope it helps,
Dan