Unpack your dimmer and connect at least one of the L terminals and the N terminal to your mains. If you are unsure about dealing with mains voltage (danger beware), you can follow me on video here or ask someone who knows more about it.

• Use your phone or computer as wifi client and connect to the AP provided by the original shelly Mongoose OS firmware.
• open http://192.168.33.1/ in your browser
• Go to Wifi Settings and change mode to Client, give it your local (internet connected) SSID and PSK and let it connect. (You will lose your wifi connection on your client here).

• Find the IP of the shelly dimmer 2 in your routers dhcp table (my example: 10.1.10.170)
• Next, since the shelly has internet now and you can reach it in your own wifi/lan, open a browser, copy the below URL, paste it into your browser and substitute SHELLYIP with the one you found and press enter.

http://SHELLYIP/ota?url=http://ota.tasmota.com/tasmota/shelly/mg2tasmota-ShellyDimmer2.zip

In this example my URL would look like:

http://10.1.10.170/ota?url=http://ota.tasmota.com/tasmota/shelly/mg2tasmota-ShellyDimmer2.zip