If you are unfamiliar with the portal file structure, please look at this article.
Portal templates is dynamic by nature. You may need to change a bit of text or some picture in it.
To get started you'll need to download the archive of the current template.
Go to the portals page of the admin console.
Choose the portal you wish to edit from the list and click on the download icon (the cloud with the down arrow) in the edit toolbar.
Unzip the archive and open the index.html file with any code or text editor you want.
the index.html file is where you should find most of the visual template of the portal.
Modify what you need, save your file and compress it again in a zip file (the best way to do it is in the folder itself by selecting all the files and compressing them instead of the containing folder).
Go back to the portals page and next to the download icon, you have the upload icon (the cloud with the up arrow). When the archive is sent, press save. Your new portal template is now online.
Complex changes may need some web development skills. If you do not feel confident you can achieve what you want, feel free to contact our Creative Lab.