Most of POS orders made directly in the store. Therefore, it does not make sense to set carrier as something else rather than "Pick up in-store". Unfortunately,if we change default carrier of PrestaShop to Pick up in-store, your online / front customers will "benefit" from the same thing while they are not in your store.
Understanding the scenario, POS Pro develops its own option to set default carrier, but only applicable for POS orders.
If it's your case, you are 1 step way from getting it done.
In your back office, locate to Point of Sale Pro > Preferences, there will be an option called "Default carrier". Just change it and done. Here is an example.
Now, let's switch to POS screen, refresh it and see how it works.
Other articles of Point Of Sale Pro: