An expense is locked when Edit is greyed out in its details, or when the edit form lets you change only one field. There are three reasons, each with its own fix.

Why it's locked | What you can still change | How to unlock it |
|---|---|---|
It's approved and Lock Approved Expenses is on | Nothing | An approver rejects it |
It's on an invoice | Reimbursable only | Remove it from the invoice |
It's part of a reimbursement | Billable only | An Administrator deletes the reimbursement |
While Lock Approved Expenses is on, an invoiced or reimbursed expense can't be changed at all. An expense that's both invoiced and reimbursed can't be changed until you undo both.
Approved expenses
Lock Approved Expenses, in Settings › Expense Settings, stops everyone, Administrators included, from editing or deleting Approved and Automatically Approved expenses. To change one:

An Administrator or Expense Approver goes to Expenses, opens the Approvals tab and chooses Complete.
They open the expense, click Reject, enter a reason and click Reject. The person gets an email with the reason.
The person edits the expense, clicks Verify & Save and submits it for approval again.
Return to Pending isn't available while the lock is on. To reject several approved expenses at once, open the older approvals page with the View Pending Approvals button in an approval email, tick them under Complete and click Reject Selected.
An approved expense that's on an invoice or in a reimbursement can't be rejected. Undo that first, as described below.
Or an Administrator or Finance Manager can switch Lock Approved Expenses off. Approved expenses can then be edited, but editing one sends it back to Verified or Draft, so it needs approving again. See expense settings.
Invoiced expenses
The edit form says "This expense is already Invoiced. You are only allowed to edit the Reimbursable field", and a badge shows the invoice number. The amount, date, category, currency, exchange rate, tax, customer, project and Billable can't change once an expense is invoiced.
To change one, free it from the invoice: remove its line from the invoice and save, or delete the invoice. Remove any payments and credit notes from it first. Voiding an invoice doesn't free its expenses. The expense is then Uninvoiced and can be edited, or rejected if it's locked. See why expenses are missing from an invoice.
Expenses in a reimbursement
The edit form says "This expense is already Reimbursed. You are only allowed to edit the Billable field". This applies whether the reimbursement is still processing or already paid.
To change anything else, an Administrator deletes the reimbursement: go to Expenses › Reimbursements, choose Processing or Paid, and choose Delete in the reimbursement's ⋮ menu. All its expenses go back to Reimbursable. Change the expense, then create the reimbursement again. Deleting only removes the record in Avaza: it doesn't reverse a payment. See expense reimbursements.
Other reasons you can't edit or delete
- Someone else's expense: only Administrators and Finance Managers can edit other people's expenses.
- The person's role: an expense can't be edited while its owner doesn't have the Timesheet/Expense User role. Give them the role again first.
- Deleting: an invoiced expense can't be deleted, nor can an approved one while Lock Approved Expenses is on. In the expense details, Delete is also greyed out for expenses in a reimbursement.
- Pending expenses aren't locked. But if you edit your own, it goes back to Draft or Verified, and you have to submit it again.