Upgrading WP 3.3.1 to current

Hi all,

Disclaimer: I have no idea what I am doing nor do I understand the technical jargon associated with web development/management.

Our current WP is on version 3.3.1 and we must upgrade for security reasons. The issue is I have no idea how to do this. A colleague downloaded the database and files for a proper backup. I’ve watched a number of videos and referred to the WP detailed instructions for manual upgrading, but am at a complete loss at present.

Clear and simple instructions would be amazing.

Thanks.