If you fixed a bug or enhanced a feature, please get in touch with someone in the community (ex. on IRC) and to ask for commit access.
If you are not sure about your fix, or if you do not know how to use SVN or otherwise can’t do this at the moment, you can submit here on dev.tikiwiki.org as a patch. A patch is a regular tracker item and you can attach your patched file(s). If it’s just a few lines, you can also just put the patch in the tracker item using the CODE plugin.