This can be done, but not within tmsh. If you drop to the unix shell ("run util bash" from tmsh), you can use standard unix utilities to edit the /config/bigip.conf file and change the name of any element you choose (when you're done, type "exit" to end the bash session and return to the tmsh prompt). Then load the modified file (from tmsh, "load sys config verify", then "load sys config" after the validation succeeds), and then if you have an HA cluster, sync it to the other units (run cm config-sync to-group $GROUPNAME) after confirming everything's working.
Note that if you're changing the name of a node, pool, or other element that's referenced elsewhere in the config, you'll need to make sure you rename all references to it as well to match, otherwise the validation will fail.