[ubuntu] How to resize partition of Ubuntu guest OS after increasing virtual disk size

I want to increase the size of the virtual disk for an Ubuntu (XUbuntu) guest OS.
I used the VirtualBox media manager to increase the disk size from 200GB to 300GB
When booting the guest OS, dmesg and gparted still show 200GB. Refreshing disk info with gparted does not change this.

Is there something that I need to do to trigger Ubuntu that the disk size has changed?

Windows10 host OS, FYI.

Thanks!