sls deploy function command deploys an individual function without AWS CloudFormation. This command simply swaps out the zip file that your CloudFormation stack is pointing toward. This is a much faster way of deploying changes in code.
serverless deploy function -f functionName
-fThe name of the function which should be deployed
-sThe stage in your service that you want to deploy to.
-rThe region in that stage that you want to deploy to.
serverless deploy function --function helloWorld
serverless deploy function --function helloWorld --stage dev --region us-east-1