Click on the three dots on the right corner of the document you want to lock and select Lock from the drop-down menu:
As long as a document is locked, neither you nor other coworkers will be able to change the document's data such as category, department or amount.
To unlock a locked document, you or another administrator in the company must simply click on the three dots again and select Unlock:
💡 FYI! Locked documents are marked with a 🔒 icon!
Further reading
Want to lock documents automatically? Use the Lock Document action in workflow.
Want to enable another coworker to lock documents? Make them an admin.