Changing the default printer in Windows
That is great as a first tkinter application. I like the fact you wrote two specific functions update_default_printer_label() and set_default_printer(). Most programmers will just do all what you did in one function and they believe as long as it does not exceed 25 lines length they are near perfection. With your approach, you have fine grain control on your program as unit testing is simplified (because you have small functions doing one clear task).

I do not have much to say about your program. There are very few imperfections but some may argue it is Ok. Nevertheless, I do not see the reason you let the function get_available_printers() orphan (I mean it can be integrated into the class PrinterManager, it is its right place and you will add only self. to call it where you did)

I would like to add one or two extra notes: given the description of the problematic, I think it would be reasonable to create a stand alone executable for your application using pyinstaller since you are trying to avoid navigating the printer menu and, why not, schedule your program (using python-crontab) to switch to the default printer (you need to add some code to check before switching) at a specific time (for example everyday at 08:10 AM)