This can be changed if you are using the hosted version of Canvas or you are self-hosting. It might be fixable if you are using the Free for Teachers account -- I don't have experience there, but I've seen some posts that teacher's have trouble changing some of the user information (it might have been the emails).
How to do it depends on how they were entered in the first place.
If you're using SIS imports, then each person has an unique identifier and you can resend that information with the names formatted in the proper format. There is a first name and last name and Canvas will automatically combine those together for you. Here is the documentation on what the SIS Import format needs to look like.
Admins can also change it through the web interface by going to each student and editing their information. First and last name are not separated here, but you can set full name, display name, and sortable name. The "Mijares, Teresita" would be the sortable name. That is the most useful because it contains the comma to split the fields if you need to.
This discussion post is outdated and has been archived. Please use the Community question forums and official documentation for the most current and accurate information.