If I could go back in time, I would choose a different class.
If I could choose this class again, I would pick another one.
ーIf I could go back in time, I would choose a different class.
「過去に戻れるなら、違う授業を選んだのに」
go back in time で「過去に戻る」
choose a different class で「違う授業を選ぶ」
ーIf I could choose this class again, I would pick another one.
「もしこの授業をもう一度選べるなら、別のを選んだだろうに」
choose で「選ぶ・選択する」
pick でも「選ぶ」という意味です。
ご参考まで！
カナダ 
