it is not possible or even supported to rename a named instance. The only name you can change is the computername. For instance, a named instance BLOGGS on computer FRED would be

FRED\BLOGGS

You could change the computername to JOE which would produce

JOE\BLOGGS

You cannot change the instance name BLOGGS to SMITH, not with out installing a new instance named SMITH and migrating objects to it, you have been warned!