[{_id:1,"exercises":[{id:1,parameters:[{values:[1],_id:1},{values:[2],_id:2},{values:[3],_id:3},{values:[4],_id:4},{values:[5],_id:5}]}]}]

db.collection.aggregate([{$set:{"exercises":{"$map":{"input":"$exercises","as":"item","in":{"$mergeObjects":["$$item",{parameters:{$slice:["$$item.parameters",2]}}]}}}}}])