Now move to the advanced tab, simply 2 buttons left of the Startup Parameters.
Here press on the reinstall button (yes the red one) and press OK. We recommend you to download your saves before, however, we never have lost any saves during the process. It's written, All data will be lost, but only factorio binaries will be touched.
Once you have pressed wait a few seconds (10-30) usually. And you will be redirected again to the page. Go in System > Console and start your factorio server again. You will see once you have understood it's truly 2 clicks. If there are a new update and you are set on experimental (and there is one new experimental update out) press on reinstall the server. Have fun!