update collection mongodb in java

I want to update collection below with java

{
    "_id" : ObjectId("590db524e4b0c14bc0704cd8"),
    "uid" : "a528c6a2-411d-4316-b7ad-3143005122ab",
    "reportTypeKey" : "columnChart_OnPollsFinishTime",
    "data" : {
        "fk1" : "4555",
        "pis1" : [ 
            "87942d4b-2db0-4d4c-a235-b5b4486b8aa8", 
            "8ca40661-c045-4fa5-b3a7-ea2215ce5668"
        ],
        "pos1" : []

    }
} 

I want to change “fk1” : “4555” to “fks” : [“4555”]
please help me
Thanks