I have an Apple Watch Series 3, and has problems with updates. Ever time I do an update through the iPhone, I get an error that says I don’t have enough memory. I usually have to completely reset my watch.
Recently, I discovered I can update my watch directly from my watch and not through my phone. For the last two updates, it worked without resetting my watch. This last time, I got the warning on my phone, but the Watch updated with no issues.
To update your watch, go to System->General->Software Update on your watch. It must be charging and at least 50% charged. You can get to System by pressing the crown.