I’m a bit surprised that the updater app cannot handle it properly.
I would expect it to first clear the temporary download folder from previous updates (with the -f [force] option prior to fetching the latest update (and MD5 file).
But maybe the filesystem was already corrupt before the update was initiated and now the effect turns out.
Anyway it could help to eject the sd card by software settings->storage&USB->eject sd card
Shutdown the phone remove the battery & sd card. Use a matching sd card reader (probably with an adaptor µSD Card->SD Card) and run a filesystem check on a Windows console if available (chkdsk [sd card drive letter:].
If there are errors reported use the /f option.
Reinsert the card & battery into the phone, startup. see if things are better.
I’m not quite sure how to check the internal filesystem as it should be unmounted for this.
I assume using the adb tools (linux tools) would do.
Which brings me to the idea to manually installing the update/OS or maybe more comprehensive explanation.