This article assumes you have permission to administer custom data within Paid. If you do not have access please talk to your administrator.
Navigate to "Settings"->"Custom Data Fields". Click into the one you want to edit. This will have the type "Source Data"
A window will appear on the right of the screen. Scroll to the bottom.
Click the delete icon, also you can download it to see the format by clicking the middle icon.
Next, click the area to upload a new file
Navigate to your new file and select it. The new file will appear.
Click "Update" and if all is correct a green toast will show this as successful. Please make sure the format is ID, Name in the CSV.