Ubuntu 18 server not using allocated space

I'm using Proxmox and have created a VM with Ubunti18.04 server. The size of the VM is 50G, but from within the server console I'm only seeing about 8G worth of disk space, and the main partition is only 4G. I'd like to have a larger disk available to my application (Zoneminder). I've attached a screenshot.

What's going on? How do I increase the space so that more is available?

Thanks
Eric

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Attached Images

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