How to change MongoDB storage engine for replica set member from MMAPv1 to WiredTiger

I am running Mongodb-3.4x version in a Replica set environment (in all 3 different systems on AWS) i.e. One Primary, One secondary (replica) and an arbiter.

Issue that I am seeing is for some reason; the Primary node is running on "mmapv1" but the Replica (secondary node) is running on "wiredTiger" storage engine.

Now, how can I change Primary to use "WiredTiger" storage engine instead of "MMAPv1" ?

Any help/suggestion in this case would be much appreciated.

Thanks