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.