This is my first StackOverflow post so please bear with me.
What I'm trying to accomplish is a simple program written in python which will change all of a certain html tag's content (ex. all <h1> or all <p> tags) to something else. This should be done on an existing web page which is currently open in a web browser. 
In other words, I want to be able to automate the inspect element function in a browser which will then let me change elements however I wish. I know these changes will just be on my side, but that will serve my larger purpose.
I looked at Beautiful Soup and couldn't find anything in the documentation which will let me change the website as seen in a browser. If someone could point me in the right direction, I would be greatly appreciative!
What you are talking about seems to be much more of the job of a browser extension. Javascript will be much more appropriate, as @brbcoding said. Beautiful Soup is for scraping web pages, not for modifying them on the client side in a browser. To be honest, I don't think you can use Python for that.
Change website text with python