If there is no Kohen...

What if a Yisrael was Given the First Aliyah instead of a Kohen?

We are required, as a show of respect to Kohanim, to grant them precedence for all honors, including the first aliyah of krias haTorah (portion of the Torah reading). However, if there is no Kohen present, there is no obligation to go looking for one, nor is it a mitzvah to call up a Kohen who isn’t participating in the minyan—unless he joined the minyan specifically in order to listen to krias haTorah.

In a case where there was in fact a Kohen present, but he was overlooked and the first aliyah was given to a Yisrael, the aliyah should still be relinquished to the Kohen—even if the Yisrael already said “Barchu es Hashem hemevorach” which precedes the brachah on the Torah—as long as he didn’t begin reciting the brachah itself. 

It should be noted that if a Yisrael was mistakenly called up to the Torah and the aliyah was then reassigned to a Kohen, he should remain standing near the bimah and be given the third aliyah, so he isn’t humiliated by having been called up for an honor and then being given nothing. #465

https://halacha2go.com?number=465

Practical Halacha: One minute a day. By Horav Yosef Yeshaya Braun, shlita, Mara D'asra and member of the Badatz of Crown Heights.