Replace Value using a value in different column in PowerQ

Hello PowerQPro 

your code should look something like this. I just replaced my datasoure with your steps till the transformation. 

let
let
    FullPathToFile2 = Excel.CurrentWorkbook(){[Name="FilePath2"]}[Content]{0}[Column1],
    Source = Csv.Document(File.Contents(FullPathToFile2),[Delimiter="#(tab)", Columns=29, Encoding=65001, QuoteStyle=QuoteStyle.None]),
    #"Removed Top Rows" = Table.Skip(Source,13),
    #"Promoted Headers" = Table.PromoteHeaders(#"Removed Top Rows", [PromoteAllScalars=true]),
    #"Removed Top Rows1" = Table.Skip(#"Promoted Headers",2),
	TransformRows = Table.FromRecords(Table.TransformRows
	(
		#"Removed Top Rows1",
		(row)=> 
		let 
			check3Fill4 = if row[_3]<> "" then Record.TransformFields(row, {"_4", each  "----"}) else row,
			check4Fill4 = if check3Fill4[_4]<> "" then Record.TransformFields(check3Fill4, {"_5", each  "----"}) else check3Fill4
		in 
			check4Fill4

	))
in
	TransformRows

you have to update this part according you needs. These two lines should show how to get there

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Have fun

Jimmy