[header] How to replace a bicycle tire [title] Loosen the nuts that hold the axle to the frame. [step] If they are extremely tight, spray some lubricant on the nuts-a silicone lubricant or even vegetable oil will do the trick very well. ( many modern bikes will not have nuts. 
They have a quick-release which you can easily loosen and remove the wheel). [title] Release the brakes if your bike is equipped with them and they interfere with wheel removal.