To purchase extra slots for your Mumble server and/or extend the expiration of your slots, visit your admin panel > voice tab > "Upgrade Slots" tab.
Here, you will have the option of purchasing extra slots, as well as the option to downgrade your slots.
To add extra slots just select the number in the drop down and you'll be given your new price and options for discount (Dependent on length of service).
To simply pay for your current slots and extend your renewal, just select the current amount from the drop down and click "Pay for server". You will then be given a price and new expiry date.