I found a better JS RTE out there and would like to know how to replace the TYPO3 default with the one I found. I googled a lot but couldn't find anything stating how to achieve this.
This is not easily possible without "loosing" many TYPO3 features, because the rtehtmlarea highly integrated into TYPO3.
Especially the link handling is custom, so you would have to write special support to make this work.
A possible replacement is tinemce_rte. It works fairly well, but it does a little more than just replacing the editor with tinymce, e.g. it also adds custom linkhandler support. You can look into this extension if you really want to do this work.