How to change this code into private sub

In general you can do that by doing this....

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but that depends on what you mean by private....

so when you open the workbook (excel file) you want it to automatically launch this marco?....if true, change the name of the marco to Sub Auto_Open()

most definitely....you just have to choose what kind of "change" you are looking for....IE: if a change to this column or this row or this cell then run this macro....