On Changing SEO TITLES

You'll need to adjust this in the SEO panel:

https://support.squarespace.com/hc/en-us/articles/360002090267-SEO-checklist

Log in, and in the dashboard go to Marketing -> SEO.

Add your text to these areas, you may want to adjust your site title in Settings or remove the %s that pulls automatically from the site title.

I will caution you that this change will not happen overnight, you will need to wait for google/bing etc to recrawl your website and pick up the change. You can speed this up by requesting a recrawl on Google Search Console (and Bing webmaster) but it's not directly user-controllable.