I suppose you are looking for:
vagrant box update
Here is also its documentation and a quick reference (be sure to read the part in bold):
Box Update Command: vagrant box update
This command updates the box for the current Vagrant environment if
there are updates available. The command can also update a specific
box (outside of an active Vagrant environment), by specifying the
--box flag.
Note that updating the box will not update an already-running Vagrant
machine. To reflect the changes in the box, you will have to destroy
and bring back up the Vagrant machine.
If you just want to check if there are updates available, use the
vagrant box outdated command.