increase disk

What exactly do you mean? A local physical disk or a disk of a VM?
 
You can increase the disk size from the GUI. Then you will have to adapt the partitions and FS sizes within the VM. This depends highly on which OS and how it is set up. There are plenty of guides on how to adapt to larger disk sizes out in the internet.
 
I forgot to select the disk as virtual (vda1) at the time of installation.

With this configuration, how to upgrade the disk?
 
I forgot to select the disk as virtual (vda1) at the time of installation.
What do you mean exactly? The VM is not using the disk?
 
In general this works as described in my first answer. After increasing the disk size via the GUI you will be in the same situation as if you made a 1to1 copy of a physical disk to a bigger one.

The next steps depend highly on the guest OS and the way it set up the disk. You will need to increase the partitions, possibly available logical volumes and at last the file system itself.

If this is not your situation, you will have to explain in more detail what it is. Screenshots and the output of config files can help a lot to understand what is going on. :)
 

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