Gen 1 (also referred to by fans as "P1") refers to the original Tamagotchi from 1996, and to the recent remake of that device. The Gen 1 and its remake don't offer an option to resume - that was introduced in the 2000s Tamagotchi Connection line. This also applies to the Gen 2 (a.k.a. "P2").
If you mean the Tamagotchi Connection V1, then the B button isn't what you want in this case. According to
the device's manual, what you have to do is press the reset button after changing the battery in order to access the Download/Reset menu. Here's the relevant information;
As far as I know, some other later models also have similar functionality.
I hope that this is useful to you.