Hi @ISmolin,
The lead source is “the analytics source of the lead at the time of lead creation. This field is automatically set by HubSpot and can’t be modified.” You can neither edit the options not manually set it. It’s a property for reporting only. It works similarly to the original source fields on the contact object.
If you want a custom source property, you can always create one in the property settings and maintain that in parallel. The default lead source cannot be used for that purpose.
Best regards!