Without knowing your background and level of expertise it impossible to say if it is 'safe' to open your Monterra, a Garmin technician with access to the right equipment would be able to undertake this but bare in mind that as the unit is IPX7 you will be breaking waterproof seals which will need to be replaced
You don't say why you would want to replace the RAM in the unit, but my guess is either that the existing RAM is not working or you want to increase it's size, in my opinion, this would be beyond all but the most capacle electronics engineers who know how to manually replace surface mount components
I would say that it depends what you have to lose (and gain) by attempting to do this, if you have a fully functioning unit at present, I would not recommend it, however, if the unit does not work at all, is out of warranty (maybe 7+ years old) and is of no value to you in it's present state, then go for it