Hi Laksham,
Well I don't found Update deep entity method in framework, so had to go with this workaround and It saved me many time without any issues. 
As per this you will have to use POST operation only in both case (Create, Update).
Only diff will be in your Payload, while Create you don't have to pass header key(i.e. null values) and when doing Update provide the value against key Property in Payload.
Eg. You are working with SO and SO Item, so when you are creating new sales order you have to pass null values for sales document (vbeln) in your payload, so your logic will do an create operation and new sales order will be created.
and when updating sales order, pass the sales document in payload, identify it in create deep entity method and your update logic will be executed.
Once using POST, create deep entity method is triggered you can identify within code, which operations to be performed.
Ashwin Dutt REkansh Saxena Even I would like to know, if there is better or official approach for this.
Regards,
Tarun