If I travelled back in time, maybe I could develop some water and sewer techniques they didn't know about, and some basic sanitation practices.
You should read “The Variable man” by Phillip K Dick. Its a short story written in the 50s. You can probably find it for free on the Gutenburg project website.
In the story a handiman is plucked out of WWI era Europe and taken to the super specialized world of 100 years from now and creates all kinds of chaos due to his “variability”.
You could invent the flush toilet, and we’d all be visiting the Vince every morning.