You cannot!

You need to rename the user from one to another ( this moves all the documents for that user ) 

If the Rename process does not work , you will need to run this on the SQL Server to find out any unlocked documents 

SELECT * FROM MHGROUP.DOCMASTER (NOLOCK) WHERE OPERATOR = N’JSMITH’ OR AUTHOR = N’JSMITH’ OR LASTUSER = N’JSMITH’