I agreed to review a paper but something has come up and I can't complete it

You can cancel the review even if you already agreed.

To cancel, see I agreed to review an article but realized I have a conflict of interest. How can I cancel? for the process (except that you choose the "Scheduling Conflict" option).

Alternatively, you may be able to extend the review deadline. See I received a peer-review request but need more time. Can I extend the review time?

See also:

• How does the JMIR peer-review form look like?
• I received a peer-review request but need more time. Can I extend the review time?
• I agreed to review an article but realized I have a conflict of interest. How can I cancel?
• I agreed to review an article but after opening the submission I realized I am not qualified at all to review the paper