In the “Enter journal entry” screen change the “standard” to “void” type. 

I do it by making a reversing entry -- basically the same entry you already entered but with the debits and credits going the other direction.

If you use the reversing entry type you will have the original entry automatically posted again on the next month. 

Carolyn Zeak-

Although you can not delete a Journal Entry, you can void the entry by going into GL, Entries, Post to Journals.  For the Type of Entry, select Void.  If you are unable to select Void, then just post an offsetting journal entry.

For full instructions on voiding a journal entry, please see Knowledge Base article:

How do I void a General Ledger journal entry?

Sincerely,

Coleen