How do you change a repository description on GitHub?

Click on the Edit that comes when you hover your mouse over the description and project url section

They changed the looks slightly, the "Edit" button is seen when on the repositories main page, which is the "Code" tab. Here is a screencast/animated-screenshot:

As of 2020, if you chose the new design in feature preview, meta-information about the repository can be changed by clicking on a cog icon in the right-hand side menu's "About" section:

Upon doing so, a popup will appear where the description, website, topics, and homepage settings can be configured: