the blue button . . . to the best of my knowledge . . . is not really a button.
Give the heater at least 24 hours to fully heat the tank and let it stabilize itself. Is the heater fully submerged? I have found that they are off considerably when not fully submerged.
If after 24 hours it still is not close to where you set it, then return it for a new one. THere will definitely be some variation between actaual temp and the setting. (+/- 2 degrees) Anything more and I would exchange it . . . you never know what else could be wrong with it.