If you have your root partition on LVM, you can use lvextend -L+<size>G <vg>/<lv> followed by resize2fs /dev/<vg>/<lv> (assuming an ext[234?] root filesystem; the appropriate alternative command should be used if you're running a different filesystem).
On the other hand, you may not be using LVM for your root filesystem, in which case... who knows what you need to do? Certainly not us.
If you have your root partition on LVM, you can use
lvextend -L+<size>G <vg>/<lv>
followed byresize2fs /dev/<vg>/<lv>
(assuming anext[234?]
root filesystem; the appropriate alternative command should be used if you're running a different filesystem).On the other hand, you may not be using LVM for your root filesystem, in which case... who knows what you need to do? Certainly not us.