If I haven't misunderstood the question, here is another relatively straightforward approach:

 data[All, <|#, "LOCATION" -> If[#PLATFORM == "Dropbox", foo[#LOCATION], #LOCATION]|> &]

Does this do what you want?

Here is another relatively straightforward approach:

 data[All, <|#, "LOCATION" -> If[#PLATFORM == "Dropbox", foo[#LOCATION], #LOCATION]|> &]

Or using Mr.Wizard's suggestion:

data[All, <|#, "LOCATION" -> If[#PLATFORM == "Dropbox", foo, # &][#LOCATION]|> &]

If I haven't misunderstood the question, here is another relatively straightforward approach:

data[All, Append[#, "LOCATION" -> If[#PLATFORM == "Dropbox", foo[#LOCATION], #LOCATION]] &]

It can potentially reorder the columns, although perhaps that's not an issue?

If I haven't misunderstood the question, here is another relatively straightforward approach:

 data[All, <|#, "LOCATION" -> If[#PLATFORM == "Dropbox", foo[#LOCATION], #LOCATION]|> &]

Does this do what you want?