There's a couple of ways to open files within SharePoint, and each way has its pros and cons. 

Doing this is much faster than downloading the file to your machine and opening it using its respective program. You can view it instantly from within the web browser you're using. This saves lots of time, especially if you're strictly viewing the file rather than working on it. If you do choose to work on it, you'll be sacrificing the broad functionality you get by using the full version. For example, if you choose to work on an Excel spreadsheet from within SharePoint, you won't have access to the same level of complexity with its functions as you'd expect from using Excel. If you require this level of functionality, you'll want to open files from your desktop.

As mentioned above, doing heavy amounts of editing on a file may require you to save a file to your local computer and open it from there. Here's how to do this:

From within your team site, look for the file you want to download. Hit the  icon and then select 

The file will appear at the bottom of your web browser once it has completed downloading. By default, files you download from SharePoint will be available in the Downloads folder on your PC. You can drag and drop this file to any location as you please, and will be able to open it with the program it is compatible with. If you can't find a program to open the file with, feel free to raise a ticket on the Support Portal and we will assist.