Battery light has been going on but later turns off?

Battery light has been going on but later turns off. Recently noticed radio turns off than back on. We first thought it was our battery, took it to autozone and was told battery was good. System test passed. Took it back this morning and was told it was the alternator. My question is where is the alternator on my vehicle and how hard is it to replace. Any help would be appreciated, this is our only vehicle.