Over time, the virtual hard disk of your virtual machine will fill up more and more and it will become necessary to enlarge it.
However, enlarging the virtual hard drive will also require enlarging the file system located there so that you can benefit from the new capacity of your virtual hard drive.
Which is quite simple on recent versions of Windows.

1. Current virtual hard disk size
2. Extend virtual hard disk
3. Extend virtual hard disk file system on Windows

1. Current virtual hard disk size

Currently, our "Win 10 v1909 x64" virtual machine has a 32 GB virtual hard drive.
Note that the provisioning type used does not matter, except that the added space will be directly pre-allocated if the current provisioning type is "Thick Provision ...".

As you can see, from the guest OS we can also see that the disk has a capacity of around 32 GB.

2. Extend virtual hard disk

To enlarge (expand) your virtual machine's virtual hard disk, turn off your virtual machine, then click the icon: Edit Settings.

As you can see, our virtual machine's virtual hard drive is currently 32 GB in size.

Increase the indicated size as desired.
In our case, we indicated "40 GB" instead of "32 GB".
Then click OK.

Note that you cannot reduce the size of a hard drive by specifying a value smaller than what was previously specified.
In this case, this error will be displayed: Enter disk capacity which is larger than its original capacity.

It is therefore important to start with a small virtual hard disk, then expand it over time. Since you will not be able to go back (try to reduce the size of it).

Now, our server tells us that we have a 40 GB virtual hard drive.

3. Extend virtual hard disk file system on Windows

Now that your virtual machine's virtual hard disk has been extended (enlarged), you also need to enlarge the file system available on it from the guest operating system.
To do this, start the virtual machine.

Open File Explorer and right-click "Manage" on "This PC".

Then, in the "Computer Management" window that appears, go to: Disk Management.
As you can see, unallocated space appeared at the end of the extended (enlarged) virtual hard disk.

On recent versions of Windows, it is possible to extend a volume (or partition) by right-clicking "Extend Volume" on it.

The Extend Volume wizard appears.
Click Next.

Leave the size shown as default and simply click Next.

Click Finish.

The volume has been expanded and the added disk space is now usable.