After checking with the company, we decided that we should delete all journal entries recorded during the period 1-6 / 2018. What is the right way to delete so many JL?

I can help you delete multiple journal entries, alin-shelef.

For now, you'll need to delete the journal entries one at a time. 

1. Go to your Chart of Accounts.
2. Select the bank account.
3. Select a journal entry.
4. Click on More and select Delete.

Please note that if these journal entries were already reconciled, deleting them will affect the bank balance. It can also ruin your company's financial report. To make sure that won't happened, you'll need to unreconcile the previous months transactions up until the period 1-6/2018. Delete the journal entries and then reconcile again. 

Post more questions here, and we'll be happy to answer you.