Backup all databases, script out all server wide objects, uninstall, reinstall, run all server with objects back in, restore all databases bar master
Or
Install a standard named instance, transfer everything to that instance.
Uninstall Enterprise
Install Standard
Transfer from Named Standard to Default Standard
Uninstall Named Standard
Or
Get another server
Install Standard
Transfer all objects
Shut down old server
Rename new server
Drop old server name sp_dropserver
Add new server name sp_addserver