What happens if I come home from abroad a day before the interview? Will this cause any problems during the interview or will they just update the N400 form?
No problem at all. Just inform the officer about your travel after you send in your application up to your interview. You N-400 will be updated accordingly.
But will the procedure be different because I arrived the day before, or it doesnt make a difference?
No, it doesn't matter. You can travel before the interview and there is no problem if you do not break the continuous residence or physical presence requirement. The only problem would be if you do not show up for your interview. Other than that, you can arrive 1 millisecond before the interview from abroad. The important thing is that you need to inform the officer about your latest trip(s) so that you N-400 can be updated accordingly.
What does "updating the N400" mean? Will the officer have my application infront of him/her and then add the info to it?