Sorry to hear the calendar is having issues. I would suggest you try the following and see if it helps:
1) Go into your iPhone settings and select Reset cache on next launch- https://community.canvaslms.com/t5/Canvas-Student-iOS-Guide/What-Student-app-options-can-I-manage-fr...
Note that this will also remove your login information, so you must log in again.
2) If #1 doesn't work, you should delete the app in your iPhone storage settings (Go to iPhone Settings > General > iPhone Storage > Find Student app and select Delete App). If you delete the app from your home screen, it might not completely remove all app files.