As SLV said,
1. You can directly download into the firewall and then install it.
2. Download the software on your laptop/desktop from https://support.paloaltonetworks.com/Updates/SoftwareUpdates/245 and upload it manually into the PAN firewall. ( scp import from CLI as well)
But, USB port is currently not supported and please find below related feature requested that already submitted:
Load firmware and weekly updates via USB
FR ID: 2511
Load a configuration via USB memory stick
FR ID: 841
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