The Subscriptions extension is using layouts often. The benefit of layouts is, that there is a small pieces of code that is easy to maintain, and even better to make an override for it. When searching the internet you don't find that mucht about layout overrides for Joomla! extensions.
In summary, to override a layout, you have to copy the specific layout file to the template folder (of course the active template) and manipulate the copy for your needs. Joomla! searches for a required layout file the template folder first. If found, this one is used, else the one at the extension layout directory.
There is a huge benefit to do so. Assume you have modified the layout file at the original extension folder. After the next update, all your modification will be gone, cause the update has all of them overridden.
So let's do it step by step. Below you can see the folder structure of the Joomla! installation. Inside the red borderd folder "layouts", all laout files reside.
Here are the layouts for the subscriptions rendering.
In summary, we have a source "layouts" folder.
and need to copy the layout file wich should be modified to
The override folder
/html/layouts/com_lartssubscriptions/ has to be created by yourself.
Copy the files you want to modify into that tempalte subfolder and the layout would be rendered with your changes.