how to dynamically replace values for multiple columns

Hello vc25, 

You can dynamically replace values in all columns that start with "mt" by using a lookup mapping and Table.TransformColumns.

Step 1 – Create a mapping table
Create a small table with your replacements and load it into Power Query, for example call it StateMap:

Code | State
1  Georgia
2  Alabama
3 Texas
4 Florida

Step 2 – Use this M code in your main query

let
    Source = YourPreviousStep,

    // Get all columns that start with "mt"
    MtColumns = List.Select(
        Table.ColumnNames(Source),
        each Text.StartsWith(_, "mt")
    ),

    // Convert mapping table to record
    MapRecord = Record.FromTable(
        Table.TransformColumnTypes(StateMap, {{"Code", type text}})
    ),

    // Replace values dynamically
    Replaced = Table.TransformColumns(
        Source,
        List.Transform(
            MtColumns,
            (col) => {
                col,
                each Record.FieldOrDefault(MapRecord, Text.From(_), _),
                type text
            }
        )
    )
in
    Replaced

Why this works
It automatically finds all columns named mt1, mt2, mt3, etc.
It replaces their numeric values using the mapping table.
It works even if the number of mt columns changes.
Everything is done in one single step, no hardcoding.

This is the standard scalable pattern for dynamic value replacement in Power Query.