As a salesforce developer, many of them might come across this requirement to edit the converted leads. Yes, with Spring 16 release, now you can update the converted leads.
We have to setup few permissions to do think so.
1) From Setup, enter User Interface in the Quick Find box, then select User Interface.
2) Select Enable "Set Audit Fields upon Record Creation" and "Update Records with Inactive Owners" User Permissions and Save your changes.
3) Add this setting to your profiles. From Setup, enter Profiles in the Quick Find box, then select Profiles. Select the profile and then select Set Audit Fields upon Record Creation and Save your changes.
Yes, now the users with these profiles, can edit Converted Leads.
If you want to update Converted Leads from data loader. Then you should consider the following points.
1. In Data Loader, navigate to Settings -> Settings.
2. Update Batch Size = 1.
3. Attempt update operation.
If you try to do bulk update or without set batch size = 1 then you will get this error - "ERROR: insufficient access rights on object id".
Happy Coding !