This simple. :thumbupI cant see that link it tells me Sorry - no matches. Please try some different terms.
but it is a pretty easy fix on my 91 I pulled the hoses off the back removed the glove box lid un hooked the blend door cable to make access to the screw by it easier to get to pulled all the screws removed the cover pull the heater core out replace the new one and put everything back together. it took about 15 minutes start to finish minus the time refilling the cooling system I did a flush at the same time