Changing your VPS' root password is very simple! Below are the steps you'll need to follow:
1) Go to My Services and click on the relevant service
2) Scroll down the page and click on the green Manage my VPS »
button [screenshot]
3) Click on Root/Admin Password
and type your new password in, and click on Change
[screenshot]
The root password should have now been changed, without the need for the VPS to be rebooted