Replace certain weeks in months Power BI Query

Remove last 2 lines in your code i.e. 

in
#"Duplication de la colonne"

Put a comma at the end of line #"Correction orthographe Extract_Month"

Paste following code after #"Correction orthographe Extract_Month" line

    FilteredOnDates = Table.SelectRows(#"Duplication de la colonne", each not Text.StartsWith([Extract_Month], "W")),
    Weeks = Table.SelectRows(#"Duplication de la colonne", each Text.StartsWith([Extract_Month], "W")),
    SecondLastDate = List.First(List.LastN(List.Sort(FilteredOnDates[Extract_Month]),2)),
    MinW = List.Min(Weeks[Extract_Month]),
    Custom1 = Table.ReplaceValue(#"Duplication de la colonne",each [Extract_Month],each if Text.StartsWith([Extract_Month],"W") then if [Extract_Month]=MinW then SecondLastDate else null else [Date],Replacer.ReplaceValue,{"Extract_Month"}),
    #"Filtered Rows" = Table.SelectRows(Custom1, each [Extract_Month] <> null),
    #"Changed Type" = Table.TransformColumnTypes(#"Filtered Rows",{{"Extract_Month", type date}})
in
    #"Changed Type"