You can't change it from the UI. It's a server side setting. Go to the environments page, go to the details page of the environment you're using and go to the model options tab. Here you can enable/disable.
The actual setting on how often you want it to run, can only be changed in the modeler. See the documentation:
If you want flexibility: run it very regularly (every day, every hour, every minute, whatever works for your use case) and do a test in an exclusive split to determine whether or not the rest of the flow must be executed. This is also explained in the documentation in the bottom.