This is not a currently implemented setting. A previous Idea was suggested for this but it was marked as Archived, so not something Instructure is currently planning to implement either:
https://community.canvaslms.com/t5/Idea-Conversations/Military-Time/idi-p/488645
There is a way to force this, but it is not due to any setting to change the time format. If you go into your account settings and change the language to English (United Kingdom), it will cause the system to report times in 24 hour format and correctly formats it for the Time Zone you have your account set to display. Get to your User Settings by clicking on your Account icon at the top of the site navigation on the left.