[{brand:"Goat The Label","imageUrl":"https://cdn.test.com/s/files/1/0035/4671/0061/products/Screen_Shot_2019-12-13_at_12.47.01_pm_2000x.png?v=1576212956\n \n https://cdn.test.com/s/files/1/0035/4671/0061/products/Screen_Shot_2019-12-13_at_12.47.01_pm_2000x.png?v=1576212956\n \n \n"}]

db.collection.aggregate([{$match:{imageUrl:{$regex:"_2000x"},brand:"Goat The Label"}},{$addFields:{imageUrl:{$reduce:{input:{$slice:[{$split:["$imageUrl","_2000x"]},1,{$size:{$split:["$imageUrl","_2000x"]}}]},initialValue:{$arrayElemAt:[{$split:["$imageUrl","_2000x"]},0]},in:{$concat:["$$value","_850x","$$this"]}}}}}])