My aim is to have a form and on that page be able to
click a button, have the form enter the data on the
datasheet. If one text box is empty it will not write a
blank over an existing value.

I have been using the code below and it only allows me to
write over the existing cells value.

Set Rng = shData.Range("I" & Application.Match(CLng
(sh.Range("D6")), shData.Range("B:B"), 0))
Rng.Value = sh.Range("D8").Text

I have tried to adapt it and I get an error.

If sh.Range("D8") Is Null Then
End
Else
Set Rng = shData.Range("E" & Application.Match(CLng
(sh.Range("D6")), shData.Range("B:B"), 0))
Rng.Value = sh.Range("D8").Text
End If

What do I need to add?
Also where do I start a message box sayaing

"Cell "A1" already has a value of 3.22, would you like to
replace with 2.66?. Click Yes to Replace, No to skip.