You'll only exhale the CO2 you actually produce, if you increase your oxygen intake you won't actually increase CO2 production, you only do that with increased activity (that's why you need to breathe more). You can also try to hyperventilate, but it won't work for the same reason.

90% of weight loss is cutting down on intake, there are plenty of effective techniques that might work for you, but in the end the only way to lose weight is to eat less.