you can report the series with a reason of "incorrect or missing volume numbers", and state in the report which chapters belong to which volume.
it isn't exactly intuitive since no other site i know of uses a report system in this way, but on the other hand there aren't really other sites that have this weird quirk of both having objective cataloguing information (series, chapter, volume) be determined by the uploader, but also allowing multiple uploads with the same cataloguing information to be made (as opposed to wiki-style where everyone edits the same copy) - which leads to an interesting situation when someone wants to update out-of-date cataloguing information (which is universal and objective) on someone else's upload (which cannot be edited by other users). i don't blame them for hijacking an existing method of requesting review from site admins (aka a report system) for stuff like this, where manual edits by said admins are basically the only practical solution.