The firmware installer for Iridium 9555 HT19002, along with the release notes may be downloaded by clicking this link.
If you'd like to review the release notes prior to download, you can do so here:
IRDM-1023-SRN-002 v5.1 - Iridium 9555 Software Release Note - HT19002.pdf
Note: There have been many 9555 hardware revisions over the years, not all of which have been listed in the release notes (under 'Hardware Compatibility'). All revisions up to the release of the firmware update should function properly.
The HT19002 release notes contain steps that should be used to upgrade an Iridium 9555 handset from firmware version HT11001 or later.