You can rename a custom field in Marketo, but there's one catch: you must remove all of its use in the system before you do so. This includes forms, smart lists, and smart campaigns.
Admin Permissions Required
1. Go to Admin and click Field Management.
2. Find and select the field you want to rename, then click the field name in the canvas.
Click the Used By link to find assets that reference this field.
3. Rename the field and click Save.
Nice job! You now know how to rename fields in Marketo.
If you rename the API name in Salesforce, Marketo will create a brand new field and leave the old one behind!