Travel from Interlaken (Mürren) to Milan Malpensa Airport (Italy)

Mürren to Grütschalp
Grütschalp to Lauterbrunnen
Lauterbrunnen to Interlaken Ost
- all these are simply the reverse of how you arrived -

Interlaken Ost to Spiez (a stop on the way to Bern, which may be the way you came in)
Spiez to Milano Centrale
Milano Centrale Malpensa Express to the airport.

You're not planning on flying the same day you leave the Lauterbrunnen Valley are you?

I have done this trip. If you look at the DB Bahn website you will see a train leaving Interlaken West at 7:33am and arriving in Milan Centrale at 10:37am with one transfer in Spiez for 15 minutes. From Milan Centrale it is easy to get to the airport on the Malapensa Express from Milan Centrale. If the May 31st is your flight day, I would suggest doing this train trip on the 30th and spending the night in Milan to avoid rushing before a flight.

you don't have to figure this out yourself. The railway websites will choose the best routes for you, when you plug in the departure and arrival stations. It probably looks more complicated than it is.

Thanks for all of the helpful information. It will certainly make our task easier now.