If you let IRO assemble your bike they make the chain lengths super tight to the rear drop-outs, it's almost silly how tight they get them (all that is needed is 1 or 2 more chain links). The problem is even worst if you change the rear cog up from whateve ryou ordered.

Regardless, loosen the rear hub andpush the chain off the chainring. Use your finger like a front deraileur and push to the inside. It will run off with a little bit of effort. IRO makes them tight, but not so tight that you can't push it off. To reset the chain just catch one or two teeth on the front chainring and the chain will run on with little effort...THEN tighten the rear hub.