Dynamically Rename 1st Column

I am trying to dynamically rename the 1st column from whatever its Number value is to just "Column1"

From here i use = Table.ColumnNames(#"Promoted Headers") to get the column names as a list.

From there i only want the first item in the list so i use List.FirstN(Custom1,1)  and that gets me the 1st item which is the first column name

Now i want to rename that column in the #"Promoted Headers" table/step so i tried

= Table.RenameColumns(#"Promoted Headers", #"Kept First Items", Column1) and this is what i get

Not sure if i am using the function incorrectly some help would be great.

And here is the Mcode

#"Promoted Headers" = Table.PromoteHeaders(#"Transposed Table5", [PromoteAllScalars=true]),
Custom1 = Table.ColumnNames(#"Promoted Headers"),
#"Kept First Items" = List.FirstN(Custom1,1),
Custom2 = Table.RenameColumns(#"Promoted Headers", #"Kept First Items", Column1),

Thank you,