Originally Posted by Andreasaway

With your current setup you need to cross gear the bike, don't do it with load on the pedals. So once it is in the big ring in the front and the 25t in the back, take the chain at the bottom and try to fold a couple links out. If you can overlap 2 links you should have enough chain for the 27t. Good Luck
P.S. if you have had your trail-a-bike for more than about two years, you should check the website, as they have a recall on alot of them.