If you tap Change Instance it should find the post you are on on your account's instance. Eg. let's say your account is on the lemmy.dbzer0.com instance and you open a lemmy.world link. You will not be able to perform any actions by default because you are not authenticated (signed in) on lemmy.world.
However this dialog should popup when you try to perform an action giving you the option to change your instance from lemmy.world (which is the current instance since that's the instance of the link you tapped) to the instance of your account lemmy.dbzer0.com.
This might be a communication error on my part as I might not have been clear enough in that dialog. If you can think of a better way to phrase the message or the buttons let me know.