Honest question, couldn’t Biden have just changed it, as president?

I don't believe that's true. He wanted to start orderly withdrawal sooner so that it'd be well underway before the election, but his generals postponed it (and then hightailed out of there in a highly disorderly fashion leaving the Taliban in charge and giving them billions of dollars in materiel).

Biden is the commander-in-chief, yes he could have changed anything about it that he wanted to. He could have gone back to the Taliban with modifications to the terms. They were not in a strong position to resist, frankly. If the US had said they needed three more months to do it effectively, the Taliban would have agreed. Also Biden was making choices along the way related to the time table (Wikipedia covers this fairly well).

https://en.wikipedia.org/wiki/2020%E2%80%932021_U.S._troop_w...