tverticchio on "Restore old theme"

I just updated our wordpress version and then updated the theme. Nothing had been updated for probably 2 years or more. We are using the wordpress default theme. Unfortunately I didn't write down what version of wordpress or Default theme we were at.

It looks like I lost some styling and a baer picture that was part of the header. Also, we had a menu sidebar on the left, not it has moved to the right side. There is also a line (shadowing) down the middle of the page.

I am assuming this is a result of the theme upgrade. I would like to revert to the version of the default theme I had, but have no idea how to do it.

I am new to site management and word press and was not involved in the original setup and design.

Thank you for any and all help provided!