There is no difference.
(Romaji is the use of the English alphabet to spell Japanese words.)
Because there is no “m” in Romaji, the letter “n” is used as a substitute.
In Japanese, senpai is spelled せんぱい. The Japanese kanna ん corresponds with the romaji “n”. The only kana representing m sounds are maま, miみ, muむ, meめ,and moも. However, phonetically senpai is pronounced like seMpai. So, it is not phonetically wrong to spell sempai, but the proper spelling is senpai.
Senpai is what you would call an upperclassman or your senior.
