Avaliablity dates

How would you go about changing when you are avalible to work? And if you do have to change them or cant work them will they possibly fire you?

You have to call the person who hired you (should be on your contract) and work something out with them. They do have the option to retract their offer of employment if they don't agree with your new availability dates, but generally they'll try to work it out with you if the request is within reason.