How to change the RAM allocated to an OS in VirtualBox?

Pretty easy to do.

1. Power down the VM (the guest has to be off, not just not running but with a saved state).
2. Discard 'Saved State'
3. Open the VM's settings.
4. Open the System tab.
5. 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.

In addition to Dennis M, to change Vm machine memory settings via command line, I used :

vboxmanage modifyvm "Centos7_Vanila" --memory 1000

• Where "Centos7_Vanila" is my VM name.

• To Get your imported / listed VMs run following command :

  vboxmanage list vms