seeing "Release upgrade package file validation failed. File seems to be corrupted." when upgrading from 126.96.36.1994 to 188.8.131.52. Downloaded two copies and both times getting the same error saying it cannot validate the file after I drag and drop it on the upgrade page.
Confirm that the file was cleanly downloaded (not partially). If it failed on file validation, that seems to suggest that the download was partial and hence the overall signature must have failed for file integrity.
Could you please confirm you used the upgrade file (apic_em_update-apic-GA3-184.108.40.20683.tar.gz) downloaded from the "APIC-EM Software Updates" link.