If you are an extension developer and creates plugins then please read the extension plugin article.

Normally all developers saves their plugin templates in folders using this scheme:

inc/template/<extensionKey>/<pluginKey>/

A good practice is, that a developer gives you the ability to change the output template in the plugin properties as a select box. This select box contains all files in the folder above - by choosing one the plugin use this template to generate their HTML.

So, if you want to adjust the plugin output or the style, just create another (maybe a duplicate can help) file in this folder (mostly with file extension .tpl, look at the files inside the folder to know more) and adjust this file. When you saved your file it should appear in the plugin property and is ready to choose.