Unfortunately saved Data Maps cannot be edited directly in CRM. This can be confusing to users especially as they can change the Data Map mappings at import time however these changes only relate to the current import and cannot be saved.
If you need to change a Data Map then you need to export it, manually change the XML and import the changed file under a new name.
Tip: if you are using Visual Studio to edit your Data Map press Ctrl+D, E (or Edit, Advanced, Format Document) to format the XML and make the file more readable.
Also the source file name is stored in the data map and if source file name you select does not match that stored in the Data Map you will get prompted with the message Some data files are not mapped – Please map them to Dynamics CRM records types to continue. Simply select the target entity and continue.
If you have used a different source file you will get the option to save this as a new Data Map however you cannot overwrite an existing data map and if you try to do so you will get the error a map with the specified name already exists.