I'm going to change someone's instagram password for using my email
Why do you want to change someone's Instagram password for using your email instead of contacting Instagram?
I thought if someone is using my email address then the account belongs to me.
If someone else is using your email address for their account, you should report it to Instagram. Changing their password will only end up getting you in trouble instead of them.
A report is not needed, I know what to do.
If someone is using your email for an Instagram account, then they might have more information about you; maybe passwords for your email, itself. It's better to contact Instagram instead of using the account, because the account may already be considered suspicious, and it may get banned under your control.