How can I convert immutable.Map to mutable.Map in Scala?

The cleanest way would be to use the mutable.Map varargs factory. Unlike the ++ approach, this uses the CanBuildFrom mechanism, and so has the potential to be more efficient if library code was written to take advantage of this:

val m = collection.immutable.Map(1->"one",2->"Two")
val n = collection.mutable.Map(m.toSeq: _*) 

This works because a Map can also be viewed as a sequence of Pairs.

val myImmutableMap = collection.immutable.Map(1->"one",2->"two")
val myMutableMap = collection.mutable.Map() ++ myImmutableMap

Starting Scala 2.13, via factory builders applied with .to(factory):

Map(1 -> "a", 2 -> "b").to(collection.mutable.Map)
// collection.mutable.Map[Int,String] = HashMap(1 -> "a", 2 -> "b")

How about using collection.breakOut?

import collection.{mutable, immutable, breakOut}
val myImmutableMap = immutable.Map(1->"one",2->"two")
val myMutableMap: mutable.Map[Int, String] = myImmutableMap.map(identity)(breakOut)