You have to apply a credit memo to the debit memo.

Are you in AR or AP?

You can do a credit memo on the wrong date and then start again. 

Sarag-

I believe we discussed this in your support chat.  But to follow up, you cannot voied a debit memo, but you can create a credit memo to offset the debit memo.  Then repost the debit memo correctly.  For full instructions, please see enclosed Knowledge Base article:

How to void a credit or debit memo in Accounts Payable or Accounts 

Sincerely,

Coleen