You can change the name of a weapon kind, meaning for example #23 in your database, however not for individual weapons (aka renaming one of your instaces of item #23 differently than the others) without doing some work. If you just want to rename a weapon kind, though, you'd go about it like this:
$data_weapons[23].name = "Newname"# change weapon #23's name to 'Newname'
If you want to change an item that's already in possession, you'd do this:
$game_weapons[23].name = "Newname"# change weapon #23's name to 'Newname'
You can do all of these from a Call Script event command. Note that you could always get rid of a weapon and add another one instead with the same values, which would probably be handier in fact, as you have both of your weapons in the database for easy access in case you want to change them or something... It just seems like the clean way to me.