Here is an example picture demonstrating the Google Homepage edited by me to say that I own Google.
This is just an example. You can use it for anything like showing wrong traffic stats or for fooling people by showing that someone said something about them on Facebook or Twitter. It depends only on your creativity. However, I would advise you to stay ethical and use it for entertainment purposes only.
Or, alternatively you can use the following bookmarklets by dragging them to your Bookmarks bar.
These bookmarklets work on all common web browsers including Google Chrome, Mozilla Firefox, Opera, Microsoft Edge and Internet Explorer.
You can also save the modified website with your browser through the Save page as option. The editing that you do will, however, be temporary and limited only to your browser as it is not server-sided. The online webpage will not change.