You can change the name of your Mighty Network anytime by going to Network Settings > General Settings > The Big Stuff and editing the field that you see under "Mighty Network Name."
When you're done changing the name, just tap or click on "Save" in the upper right corner and this update will automatically go live across your entire Mighty Network.
On the same General Settings page, you can also edit the tagline and description for your Mighty Network, as well as change the privacy settings and configure your custom domain.