1. Open Linux terminal and run this
gsettings set org.gnome.Epiphany.web:/org/gnome/epiphany/web/ user-agent 'USER AGENT GOES HERE'
2. Restart browser.
3. Double check and your useragent is now updated
To reset the Epiphany useragent to default
1. Do the same command but with '' at the end instead
gsettings set org.gnome.Epiphany.web:/org/gnome/epiphany/web/ user-agent ''