One of the great things about having a developer account is being able to test all the new software releases early. One of the downsides however is the ease of bricking a device or regretting updating.
This happened to me with watchOS 4, the only way you can downgrade is to send it into Apple for servicing.
Here's how I did it.
First off this process isn't standard so be patient if the support representative needs to contact their manager. For me the offshore team redirected me to a manager within the Irish office. I explained that I needed to send my Watch in for restoring to watchOS 3 and once the process was found it was straight forward. So, stage 1 is call Apple Support.
Stage 2, once you have received your confirmation of contacting Apple Support via email expect a box from UPS in a couple of business days.
Stage 3, packing up your Apple Watch is straightforward and you get everything you need in the package delivered by UPS. Remember not to post your watch band back, just take it off and put the watch in the white box.
Put it all in the supplied UPS bag and head off to your local UPS Center. Mine's only a few minutes walk away but if you don't have one locally you can simply get them to pick it up from your address.
Now it's with UPS and on it's way to Apple expect a confirmation and status update the day after. This acknowledges receipt and that they are working on it.
In my experience it's worked on and sent back the same day. You'll get it next working day. Pretty amazing service, from sending to getting it back might only take 3 days.