I have already booted up an OS but find it too slow to work on. Is there a way to increase the RAM which was allocated to it initially?
Pretty easy to do.
- Power down the VM (the guest has to be off, not just not running but with a saved state).
- Discard 'Saved State'
- Open the VM's settings.
- Open the System tab.
- Change the "Base memory" value to however much RAM you want.
Remember, the VM will reserve all the RAM you allocate to it on your host, so make sure you have enough spare RAM. Most modern OSes will deal with the amount of RAM changing just fine, but you may still run in to trouble, so make a snapshot first.