The CSS on your site is still trying to load site_logo.svg, and the site_logo.svg file is still phpBB's default. So its not clear what instructions for updating the site logo have been applied to the site and "just aren't working."
For example if you had used
Knowledge Base - How to change your board logo, either the site_logo.svg file would be replaced with your desired logo, or the colours.css or another CSS file would have been updated to point to some other image file instead of site_logo.svg (like "site_logo.jpg", or "Aim Logo PHP3.gif", or similar).
Many folks replace their logo successfully with these instructions, so you're welcome to describe more specifically what you've done, and someone may be able to pinpoint what will help you get the rest of the way. You're also welcome to
post in the Wanted! forum instead, if you would really be more comfortable hiring someone to do it.