I want to lock invoices; so that no changes can be done. Now anyone can change an old invoice. How do i lock invoices so no modifications can be done?

I'm here to share some information when locking transactions, ahmed2.

The option to lock transactions can only be done once you've closed the books. Just a heads up, only the Company or Master Administrator can do this. 

Here's how to close the books:

1. From the Gear icon, select Account and Settings.
2. Go to the Advanced tab.
3. Click the pencil (edit) icon in the Accounting section.
4. Tick Close the books.
5. Enter a Closing date.
6. Select Allow changes after viewing a warning and entering a password.
7. Enter and confirm the password.
8. Click Save.
9. Then Done.

You can also use these screenshots as your visual guide:

I've also added these articles for more information when closing books:

• Close your books to lock past transactions
• Edit your closed books

Feel free to get back to me if you still need my help. Keep safe!