First thing first you need to enable debit note under Settings -> Preferences -> General:

Debit note can only be raised upon an Invoiced being approved and send to customer:

You will be able to raise debit note under three doted line button upon invoice being send to customer:

You are required to insert all the relevant details including reasons for the debit note being generated:

You are also required to specify debit note number:

1. Auto-generating debit note numbers

2. Enter debit note numbers manually

Next you are required to specify description and rate. You may also specify discount and tax if it's deemed applicable:

Now your debit note is ready to be sent to your customer.