Hi, welcome to the forum 
It would be helpful to us if you could provide some sample CSV data, and the PowerShell code that you’ve tried so far. It would also be good to know what you want to do with the changed data: are you adding it as a new column to the same CSV, exporting it to another file, passing it to another command etc.
When posting code, sample data, or errors, please use the </> button to format your code.