leejohnson wrote:
how do i unistall arch linux please.
Short answer: you don't. NOOBS is a no-frills Installer - it doesn't have tools for UNinstalling.
Medium answer: the NOOBS recovery console allows you to select the OSes to install. So by going back to NOOBS by using Shift when prompted at boot-up you can choose to install ONLY Raspbian. But note that you will erase/overwrite your existing installation of Raspbian and any files/projects that you have worked on.
Long answer: if your concern is to reclaim SD card space, then it is theoretically possible to use a partition editor like gparted to reconfigure the partitions on the SD card. If you knew how to do that, I suspect you wouldn't have asked the question...