How to change out the alternator?
My car has 126000 miles.
My car has an automatic transmission.
The correct way would be to disconnect the battery, remove the serpentine belt, and remove the lower alternator bolt. From there you would remove the cooling fan and motor assembly. Finally, you would remove the two top alternator bolts, and then remove the alternator. With the alternator removed, you can put in the new alternator and then install all of the parts back in the reverse order.
Having a mechanic perform this task will be your best bet for ensuring the alternator is replaced properly. If you are not comfortable with any of this, you can have a certified mechanic from YourMechanic come to your home or office to install your alternator for you.