To reach Múlagljúfur Canyon's access road, keep driving on Route 1 and pass by it. After a short distance, you'll come across a bridge, where you need to take a left turn (unmarked). Continue on this road and take a left at the split. Soon, you'll reach a small clearing where you can park your vehicle.
From there, hike up the glacial Moraine, and you'll reach a stunning glacial fill beach that overlooks a lagoon with a massive glacier on the other side. The lagoon and the beach are filled with ice, which you can carefully climb out on. However, I must warn you that it can be perilous, and I fell into the water myself, which was quite frigid.
What's fascinating about this place is that very few people are aware of it or visit it. It's a remarkable spot, and we had the privilege of having it all to ourselves.