You want to make mass changes to a lookup of records
Once you have your lookup go to the top tool bar then on the right hand side the second icon in is the Edit Replace icon.
If you do not see this it will need to be enabled by your administrator.
There are a number of different functions you can run using this tool such as Append text. Prefix text and update elements of text within a field.
DO NOT give this function to someone without making it clear that any changes will be "applied to the current lookup"