AWS CLI DynamoDB command

I am writing a shell script file for doing some operation on dynamoDB using AWS CLI. I am trying to update an attribute in an item in a dynamodb table if the attribute already exists. However, I am not comfortable with the syntax of the update-item command. I want to update an attribute named 'conf' with some value. However, I am not able to figure out the syntax for SET in thie command. This is what I have got till now :

aws dynamodb update-item --table-name MY_TABLE_NAME --key '{"AccountId": {"S": accountId}}'

I know the above has to followed by the SET option.

Any help would be appreciated.