The "Resign" option is available on the game info screen, either using the Settings button, or by using the "Options" menu next to your race name.

Be aware that resigning gives you a reliability penalty. If it looks like you're defeated, but you're still strong, you will get a big penalty. Consider playing the game to the bitter end. It'll help you learn, probably make a difference to the game outcome, not spoil the game too much for the others, and ultimately reduce your penalty.

See Ranking System for formulas.