I'm not 100% certain on how to replace the switches but between the information provided in my Haynes repair manual I have and inspecting my own steering wheel this is what I believe.
VERY IMPORTANT: Because the vehicle steering wheel is equipped with an airbag that could potentially deploy causing injury it is recommended that you disable the system before working on the steering wheel.
Disabling the Airbag system:
a) Turn the ignition switch to Off.
b) On Freestar/Montery models, remove the RCM fuse in the Smart Junction Block behind the driver's knee bolster (under instrument panel left of brake pedal) and watch the airbag light on the instrument panel (turn key to on position). When the light is steady (not blinking) for more then 30 seconds, this ensures the correct fuse has been removed. Turn the key to off.
c) Detach the cable from the negative battery terminal first, then detach the positive cable. Wait two minutes for the electronics module backup power supply to be depleted.
On my 2004 Freestar steering wheel, there are 3 access ports. 2 small round ports on either side of the steering wheel and one access panel on the bottom. The panel gives you access to two electrical connectors and the retaining bolt or pinion to remove the steering wheel with.
I removed one of the 2 round access ports but wasn't able to see far enough inside if there was a screw that would allow the removal of the buttons, but I would suspect so as it would be bad design to require the removal of the whole steering wheel just to remove a couple of small buttons. Give it a try and let us know the results.
If you do decide to remove the steering wheel, the Haynes manual notes that a new pinion retaining bolt is needed for re-installation so be certain to have one before removal.