how do I update this again?

Click on the Stylus icon on your browser's toolbar.
When the Stylus tab open, look in the right pane for Twitter Detox.
If there is an icon that's two rounded arrow (like a reload button), click on it to update the stylesheet
If the only icon you see for Twitter Detox is an X (to delete the stylesheet), you have autoupdate enabled!

Autoupdate should kick in SoonTM

I think it checks once per 24h or something

but yeah that's one of the upsides of using UserCSS for use stylesheets: autoupdates!