It depends which version of the SmartThings mobile app you are using.
FAQ: how to tell which version of the SmartThings mobile app that you have
If you are using the classic V2 app, then there are ways to do it. Basically you have your voice assistant turn on a virtual switch, and have that virtual switch coming on start a SmartThings routine which changes the security.mode.
However, the design philosophy changed for the newer V3 app, and now there is no way to do it. They want you to open the app and manually change it there.
See the community FAQ.