Because I want to put more plugins in my server but i need more RAM, so is there a way to change the amount i have? it's fine if I have to pay for the extra ram but I just want to know if you can or if you have to buy another server
You can upgrade the package your server is on through your billing account.
Log into https://www.beastnode.com/portal/clientarea.php and go to Services > My Services, select the server you wish to upgrade and hit the Upgrade button and on the following page select the package to upgrade to.
When you put it through you will be issued an invoice for the difference between the current and target packages, pro-rated for the days left until your next bill date. Once paid the server will need to be restarted to load it on the increased RAM allocation.