Heater matrix removal help
You do not mention if your's is a W460 or W463 chassis as the removal of the center console is different.
Any way why would you want to change the matrix, if it is not leaking leave it alone. If it is not heating check for the heater valve, these can be stuck closed in particular if left closed all summer long.
Thank you for coming back. Which model it was or is, is important to learn as the dash removal instructions would be different but as your's is not leaking it is best left alone. The task to remove the dash is big and to a degree complicated.
Whilst you do have a spare matrix, I think it would be a good idea to pressure test it for leaks any way, you could re sell it or keep as a spare.
Big, big job, does your heater work on all four speeds? As said earlier involves complete dash removal, centre consol , switch panel etc, probably two day job if you take it steady.