This example sets the value of cell A1 on Sheet1 to 3.14159.
Worksheets("Sheet1").Range("A1").Value = 3.14159
This example loops on cells A1:D10 on Sheet1. If one of the cells has a value less than 0.001, the code replaces the value with 0 (zero).
For Each c in Worksheets("Sheet1").Range("A1:D10")
If c.Value < .001 Then
c.Value = 0
End If
Next c