My odometer stopped when it hit 300,000, would like to replace it how difficult?

You probably start with disconnecting the battery to allow the airbag to discharge so it won't deploy accidentally. I take it the speedometer works that it is not a VSS issue (check engine light would be on).There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online