Whenever my mechanic has replaced the 12V in my B250e or E500W for that matter, some other electrical doodad gets messed up. So sequence may be important. Not to say the dealer mechanic will be immune but all in all, it should not take more than one hour labor so that is what you save by doing it yourself and you can choose a lower cost batter.
Examples I have experienced were: error message, sun roof in wagon would not work.
Perhaps there is a YouTube video but find one for our B250e. Remember, with ours, keep the Drive Unit and big battery safe.
Jeff in NYC
MB '21 GLC300; MB '06 E500W; MB '81 380SL
Retired: MB '17 B250e; MB '14 Be