Check the release notes for the product. If the product requires a miniroot installation, invoke Inst from the miniroot to remove the patch (the miniroot is required to reinstall the original product files, in the case); otherwise, invoke Inst from the IRIX command line.
Use the remove command from the Inst Main menu to remove the patch:
Inst> remove patchname
Note: Do not remove portions of patches; to preserve the integrity of the software product, remove the entire patch.