Deletion in B+ trees is of two kind.
1. The key to be deleted is only present in the leaf and not in the internal nodes.
2. The key to be deleted is present in both the leaf and the internal nodes.
Case 1:The key to be deleted is only present in the leaf and not in the internal nodes.
Step 1: reach the key to be deleted , since it is present in the leaf so delete it , and check if there is underflow or not.
If No underflow then Its Done.
Otherwise Patch the underflow with the two methods i.e Borrowing and Merging
In borrowing check if the immediate 2 siblings has keys sufficient to give.
Otherwise to merge.....
Tell me if you want more detailed explanation.