If you're modifying the source code, you need to redeploy the web client. Under <src>/ZimbraWebClient, type "ant prod-deploy" to re-deploy the production build of ZWC.
If you are modifying the deployed files (not recommended), then you should un-gzip the original package file (i.e. ZimbraCore_all.
js.zgz), edit the code you want to change, and re-gzip it.