How can I change my house if I was put in somewhere I didn't want?

Unfortunately, you are not allowed to change your house once sorted. The only way to change it would be to start over with a new character after deactivating your current one. Also, if you have future questions, there is a thread here that you can post them in

Just for clarification (and a small followup on Willow's post), even if you deactivate there is NO guarantee that you will be placed in that house. It is randomized based on the answers you selected for the sorting quiz.

Furthermore, in addition to Willow and Noah, you also have to wait 6 months before you can deactivate.

Also I heard that you can't get into the house you want from selecting specific questions if that house has an abundance of students in it and there are other houses that need students more. No matter what questions you select, you'll still be put in one of the houses that need students more to even the numbers out. But note, I could be wrong about this, things could have changed over time. However, this is what I was told earlier.