How to modify code in templates
Basic modifications could be made in System Config → Domains of administration backend. You can setup logo, SEO values, the text of some policy and terms pages, override CSS with your custom values. However, in some cases deeper modifications required and it can be done by making changes directly in the templates. Make sure a person who does those changes has a good understanding of HTML/CSS/Javascript and basic Java/JSP skills.
We could not provide instructions on how to make changes in the code and if you do changes on your own you are taking full responsibility for risks related to those changes. It could slow down website performance, lower security protection.
Templates could be accessed with FTP or Plesk File manager.
To find the correct path to templates please open index page and find this line:
<base href="https://<YOUR_WEBSITE>/templates/default_flat/english/" />
That means your active templates set could be found at /httpdocs/templates/default_flat/english/ folder
Making changes in templates is not recommended. If you are planning to do in-house development it is better to install additional development server and use git for version control.