Hi LauraCND,
Unfortunately, Canvas teacher doesn't yet have that function added too it, as denoted on this guide. This would generally be best placed as an Idea conversation as that feature would have to be added, and would probably fall under the banner of this theme, https://community.canvaslms.com/t5/Canvas-Themes/Increase-parity-between-mobile-and-web-versions-of-....
I'd recommend making an idea conversation and referencing that theme, but in the meantime your teachers will have to use the web version of Canvas to adjust assignment submission types. Also they can access the web version of Canvas through their phone's stock browser (Chrome for Android, Safari for Iphone), if they are in a time crunch and have no other options like in your example.