Are your agents putting lead sources in your platform like "My Aunt Sally" and you'd rather all those kinds of leads be under 'Agent Sourced' for instance? This will help prevent duplicate sources from showing up due to variations in spelling, self-created sources, etc.
This will add the entry to the list source list.
Repeat for all lead sources you wish to create and make available in the Source dropdown for any manually entered lead.
This list is then available in the 'Add Lead' modal:
Mapping Existing Lead Sources to Normalized Sources
Now that you've created your list of allowable sources, and going forward your team can only select from the list you've created, you may wish to clean up your previously existing sources. To do so, you'll map them one at a time to their new source. To do so:
1 - Select the "No Mapping" dropdown to the right of the lead source you wish to map
2 - Select the Normalized Source you wish to map to from the drop down
3 - Select 'Update' and any leads with that lead source will be correctly mapped to your Normalized Lead Source