To cancel, you will need to call 866-347-3292. You may have to pay an early termination fee if you're still under contract. See http://legal.hughesnet.com/ServiceTermsAndConditions-current.cfm
To avoid equipment charges, you will also need to return the equipment (modem, power pack, and the radio from the dish). You will get a kit to return the equipment when you cancel the account.
Keep in mind that you can always ask for help for your speed issues in the Tech Support section, as well.