He doesn't mean the client-side .wz files: he means that you have to actually edit the .exe client, as it's likely to contain the level cap value hardcoded in different places. To do that, you're gonna need some patience, trial and error, and a few tools to inspect and edit the client assembly code.
The basic concept is, the client will have two possible ways, to use a specific value for a specific situation: either by grabbing it from an external file (for example item IDs, skill graphics), or by having it directly written in the client code itself (opcodes, skill IDs in some cases).
While you can alter the external-file occurrences of the level cap by simply editing the .wz files (if there's any) and, ofcourse, the server-side occurrences of it, you will also need to check for any possible hardcoded reference (basically, things like if (level == 200) instead of if (level == variable_obtained_from_packet_recv_from_server) ).
As a hint, you might wanna search, in the software IDA, for all the occurrences of the value "200"; there's likely to be a lot of them, and ofcourse not all of them will be about the level cap, but you can then use all those addresses to test with breakpoints in OllyDbg.
There should be good guides about both of these programs, should you need to learn them, here on Ragezone.