Credit Note

No need to issue Debit Note

You can adjust ITC in the month of June, 2018 - GSTR 3B return

Yes...

Better to issue Debit Note...

Also You can adjust in Your ITC values. The Credit / Debit note not under Your GSTR 1 return...

Under GST regime, power of issuing credit or debit note lies only with supplier. In your case, you may issue debit note. But your supplier ought to file credit note issued by him.

Where is it bro?

In case purchase return then "How to send material to Supplier end and How is account by buyer?

it will be better to issue debit note.

Relevant section of the act is section 34. For accounting purpose, recipient can issue credit or debit note. But as per CGST law, the duty to issue or rather to file in return both credit and debit note lies with supplier. The law has placed the duty on supplier. This is in sync with GST return filing procedure, where credit of input tax is made available on filing of GSTR 1 by supplier. Though those provisions are not in practice exactly today, in due course of time it will become.