Change website template

I have wanted to change the look of my site, to try and get the site headder to be central and then change a few things up, But I realised I have no clue how to do this, then I started to look into it and found absolutly nothing.
so I decided to try and change the template since I made everything myself, I saw squarespace had videos on how to change it but the functionality is nolonger there at all.
To me I am functionally stuck with the header I chose when I started my website and now unable to change it.
Maybe I have just completely missed something But I think its quite silly that its this difficult to change a core part of the website.

I am increadibly stuck

@EmmetC What version of Squarespace are you using? Here's how to check: https://support.squarespace.com/hc/en-us/articles/115005308187-What-s-my-site-s-version-and-template

I would like to know if the website i'm building can be scrapped and I can start with a different template. I can't find any info on this topic.  My site hasn't even been published just yet. How do I contact customer service to speak with someone?