You can use the resize API to add a different flavor to that VM. Check this doc

http://docs.openstack.org/api/openstack-compute/2/content/Resize_Server-d1e3707.html

This is a user initiated action. You cannot set a threshold and resize it automatically on reaching that threshold.

To set threshold and the increase the size of the VM, you can do the following

HealthNMon, a opensource “Cloud Resource Monitor” provides among other features, a feature for fetching CPU utilization of a VM launched in Openstack

https://github.com/stackforge/healthnmon

You can write a custom script/cron job, which keeps checking the CPU utilization using HealthNMon, and when it reaches the threshold, call the resize API and change the flavor