Hi,
Please find link to download the firmware for this Access point, hope its a standanone AP.
https://h10145.www1.hpe.com/downloads/SoftwareReleases.aspx?ProductNumber=J9427B
Release notes in this link will have the procedure to upgrade the firmware.
Please note that it may not be a direct upgrade from existing verion to the latest, read the release notes for upgrade restrictions.