First, if you'd like to troubleshoot your speed issues you should start a new topic in Tech Support. Some speed testing will be a part of that process, the directions for which are here. Make sure to read the instructions thoroughly before running the tests and then run them as instructed, as tests run incorrectly won't be of value for troubleshooting. Also, only run the speed tests is you are NOT out of data, as speed tests run while out of data and throttled won't be of value.
As for changing your plan, you have a few options. You can email them, chat with them or call them at 866-347-3292. For email or chat, click on the "Contact Us" button on the upper right of this page.
You can also do it online in the following way...
If you sign into the HughesNet MyAccount site, then hover your pointer over Shop/Upgrade and click on Plans & Pricing, you should be able to see what plans are available to you. You can then click on what's shown to see the prices, and if you want to downgrade your plan you should be able to do so by adding the new chosen plan option to your cart and completing the transaction. Any prorated charges or credits will then appear on your next bill.