How to change several columns names using a formula?

Hi 3xc3l - this will do what you want.

 

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSlTSUTKE41idaKUkIMsIiI1BtBFYKBnINIEKmyjFxgIA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Test = _t, #"20210101" = _t, #"20210201" = _t, #"20210301" = _t]),
    ColumnNamesToList = Table.ColumnNames(Source),
    NewNames = 
        List.Transform(
            ColumnNamesToList, 
            each 
                if Text.Start(_,2) = "20" 
                then Text.End(_, 2) & "/" & Text.Middle(_, 4, 2) & "/" & Text.Start(_,4)
                else _
            ),
    CombinedLists = List.Zip({ColumnNamesToList,NewNames}),
    RenameColumns = Table.RenameColumns(Source, CombinedLists)
in
    RenameColumns

 

 

It turns this:

Into this fully dynamically. It looks for columns that start with a 20 assuming those are the years.

 

However, I am not convinced this is the best approach, but I don't know your data needs. Power BI will work MUCH better if that data is in a single column:
In my original data, I selected the TEST column and then Unpivot Other Columns.

 

 

Now just do normal data manipulation (like above with the Text.* functions) to convert that attribute to a date, the value types, then it doesn't matter if you have 2 columns or 200, all of your values are in one column and dates in another, and it will make your measures much easier.

 

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.