You will need to first download and install Gear Watch Designer which will work on PC or MAC. I recommend you use the getting started link from Samsung which explains what you need to do.
This procedure is written for the latest version of GWD 1.3.0 which was released just today.
First, you need to enable WiFi on your watch. Only needed while doing this transfer. You can later turn WiFi back off. Go to Settings / Connections / WiFi. If the large dot on right is grey, tap it to turn on WiFi. The dot will be green when on WiFi.
It is also suggested that you turn off Bluetooth.
Also go to Settings / Gear info / Debugging and turn On debugging.
You need to ensure that both the watch and computer are on the same WiFi router. You can check the IP address on both to see that they are on the same network (such as 192.168.1……)
In GWD design your watch face and save.
Then do a Build. I suggest you change the Security Level from High to Moderate. I have a new watch but I get errors that my watch is not up-to-date, so I have to go back to this step and build with a medium security setting.
At some point you will likely to asked to obtain an Author Certificate. Do that. It is easy and free.
After the build, you will also need a Distributor Certiificate. Do that. Again, it is easy and free. In the Design Ceriticate window there is a entry item for Connected Device. This should display something, (looks like garbage), if you are connected to your watch. If this field is empty, you may be able to enter the watch IP address and it may still connect and transfer.
Make sure that your watch is on and awake (screen visible) Then select Run on Device. You should see a pop-up window that shows progress. To keep my watch awake during the transfer, I touch the watch face and move my finger a bit back and forth, as if to switch screens. When the transfer is competed, the screen goes blank for a moment and then your new watch face should load. It is often reported that the new face will not remain until you power your watch off and then on.