Your Jitterbug Phonebook can be edited on the phone or on the web. To do so online, you have to go to GreatCall.com and register your Jitterbug phone number and choose a password. Once you have logged in to your account, you have access to all your account information including the approximate number of minutes you've used this month, your bills, what services you've signed up and you can even see what other services are available to you.
At the top left will be a button called "MyPhoneBook". Click on that button.
You can add and delete contacts. You just have to click on a name and hit the red "X" button.
You can also re-order the contact list so that the most frequently called numbers are at the top. You just click on a name, and press the up or down arrows above and below the red "X" button.
You can also organize them alphabetically instead, or do a combination of both - mostly alphabetical but with a couple of frequently called numbers moved to the top.
There are three small drawbacks. The first is that you can have only 50 contacts in your phonebook. This is lower than most phones these days and could definitely be an issue if you call a lot of people.
(Update: This restriction has been removed in the Jitterbug5.
The second drawback is related to the first in that each number is a separate contact. So If you wanted to store someone's home phone number and their cell phone number, you have to do it as two different entries.
If you have to do this, then you will actually be able to store the numbers for less than 50 people.
Another one I recently noticed is that you cannot you cannot easily add numbers from your Call History to your Phonebook. When you bring up your call history, the only thing you can do is call the numbers.
So in order to add it to the phonebook you would have to remember it or write it down and then switch to the Phonebook to add the number.
This is one of the tradeoffs involved in having a simpler User Interface.
Still, the capability of editing the Jitterbug phonebook online is extremely convenient. If the person using the phone is unable to update their phonebook themselves, they can ask a family member to do it online for them.