Google Map

HI
Very new to WordPress! Not sure what version I’m on but it’s not the latest. I have read the previous posts on inserting google maps but unfortunately they are not helping me. Please bear with me :o)
I have inherited a site, which has a Google Map on it, but I need to replace the map because it is not accurate. The map is on our Contact Us page which is displayed as 2 sections, the map on the left of the page and our opening hours etc. in a panel on the right. If I View Source this page, I can see the map HTML in the content section.
If I go to our Contact Us page in WordPress though, the ‘page’ only contains the opening hours text, there is no reference to the map. The only place I can see reference to it is in the Advanced Options -> Custom Fields where someone has entered a key as wp_js_file and the value as /javascript/google-maps.js
I’ve gone to google maps and got the new HTML I need, but don’t know how or where to insert it.
I know there are plugins I can get, but I don’t really want to escalate to these until I understand the basics of how are site was built.
I would appreciate any patient sole helping me!
Thanks
K