I can’t update watchOS 5.2.1

Follow (exactly) the steps in Update your Apple Watch - Apple Support. If that doesn’t solve your problem please post back with a better description of how it fails (what you do and see with your eyes) because we don’t have a crystal ball here to see what went wrong.

julieda wrote:

You indicate you have the series 1.....are you sure? If you have the Original Watch you cannot update beyond your current OS.

Maybe! There is certainly a lot of confusion about Series 1 which can update to v5.x and original release First Generation (before Series 1) which can only go to v4.3.2. A check against Identify your Apple Watch - Apple Support might be good advice.

The other frequent mistake is trying to update the watch without first updating the paired iPhone.

You indicate you have the series 1.....are you sure? If you have the Original Watch you cannot update beyond your current OS.

Excellent advice Branta.