Power Query M formula language standard library: Table.TransformColumnNames

let
   tab = #table(
    { "id", "grp", "val a", "val b"},
    {
     {  1 , "x"  , "ABC"  ,    1.3 },
     {  2 , "y"  , "DEF"  ,    4.0 },
     {  3 , "z"  , "FGH"  ,    2.7 }
   }),
   tab_ = Table.TransformColumnNames(tab, each Text.Upper(_))
in
   tab_