Hi Pino,
You login with SSH account "ampadmin" you can remove some old files.
Also read the Airwave Server Sizing Guide for the right sizing:
https://support.arubanetworks.com/Documentation/tabid/77/DMXModule/512/Command/Core_Download/Default.aspx?EntryId=38542
For VMware 500GB should be recommended to max. 750 airwave devices.
Disk expension can be done but i will advise you to work with TAC support for doing that.
Or..... Create and export a backup, install a new virtual server and put the backup back. That is most likely the fasted method.
One more thing: Start with version 8.2.10.1 the operating system is upgrades from CentOS6 to CentOS7, to get into CentOS7 a re-installation is also required.
Only put a backup back from the same version!