To do this you need to switch the server into the rescue mode, go to "My Services", then choose the service you would like and use the "Netboot" button. Change it into the rescue-pro and reboot the server. After a while you will get the rescue mode data on your email. Log onto the server using tis data. Now you have to mount the root partition (/). Here is a tutorial how you can do this:
Now you need chroot it, use command:
when you mount the root partition and chroot it, you must only type commands to change the password
Type the new password twice. Now you can switch the server into the normal boot, reboot the server. When the server is online in normal boot you will be able to use the new password.