Help! Can no longer access dashboard after changing file custom header.php

Hello Everyone,

I was trying to change the size of my header in the Structure theme and seem to have royally messed up.

My website is https://hellokirsten.com/hottopic/ and as you can see is thankfully still there.

However I can no longer access the dashboard. When I try I get the following message:
Fatal error: Cannot redeclare header_style() (previously declared in /home/hellokir/public_html/hottopic/wp-content/themes/Archive/includes/custom-header.php:9) in /home/hellokir/public_html/hottopic/wp-admin/custom-header.php on line 14

I deleted the file customheader.php from my CPanel, made a minor change to it, and then re-uploaded it. Resulting in this message. I deleted that file and reuploaded the original but the message remains. I would really, really appreciate some help!

Here is the tutorial I was following: https://sonyalynndesigns.com/how-to-change-structure-themes-custom-header-size/