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.
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