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