Database
[{"key":1,"@timestamp":"\"2022-03-09T10:43:46.000\""},{"key":2,"@timestamp":"\"2022-03-09T10:43:46.000\""}]
Query
db.collection.update({},[{$addFields:{"@timestamp":{$dateFromString:{dateString:{$substr:["$@timestamp",1,23]}}}}}],{multi:true})