Hi Logan,
I struggled with this one a bit. Let's see if I can help.
First, you need to capture the deviceID during the initial registration, and store it in a local variable.
Then in the mapping for the update, you need to map the local variable to the deviceID item in the URL of the REST service.
Are you using AngularJS? I'm updating a custom variable (which holds an array of integers) in my app. If the above tip doesn't work, I can share my code.
Hope that helps,
Andy