Deleting incorrect bill or invoice

Hello, 

I'm new in Myob. 
Just started using 2 months ago. I remembered I was able to delete invoices or bills entered

now I can only reverse sale or purchase.

Anyone can please tell me why it's happening?

I'm not sure if the system change or I clicked or unclicked something.

please let me know. 
I aslo use to change any alreary existed bill or invoices except company name but now I can't edit it at all.