Hi Anhar- Julianne from Mixpanel Support here!
You can add user profile properties in a variety of ways (ie, via the UI or the Engage API). The Users doc has more information about each of the methods. When uploading the property, you'll want to make sure to use the same '$distinct_id' that's already associated with the user. If you're updating the total orders as they're placed, you can use the mixpanel.people.increment() call.
One thing to note about user profile properties is that they generally don't contain historical values (unless uploaded via warehouse connectors as a history table). This means that you won't be able to see the total order number changing over time, just the current value. If you want to see the value over time, it would be better to use events instead.