In Glacier, you can only upload and delete files. There is no such thing as modification or overwriting. It's due to the fact, that (most likely) magnetic tape is used on which you can only write data serially. You would have to delete the old files and uploaded the modified ones again.
However, that's not really an issue as there is no need to write changes into the raw file. Even when working with DNG where the XMP is embedded in the file itself. All information is stored in the catalog.
Personally would be to convert to DNG during import and write the most important metadata, like copyright information, geo data, Keywords, and such into the DNG file before uploading.
EDIT: Forgot to mention, that there's a charge of $0.03 per GB for data deleted prior to 90 days.