I have two lists, A and B. I want to create new item in list B once I submit an item in list A. But, I want to check first if the item already exists in list B. If it exists, just update the existing item or delete it and create a new item instead. can I achieve this using workflow?
Yes, that should be doable. If this is a list workflow, I assume there is one field in the current item (Let's say "LookupID") that is the unique key (let's say "Title") for the second list.
Set a workflow variable to retrieve the ID of List2 where "Title" equals "LookupID". If the variable contains a value, you can use it to update the existing list item, else create a new one.