It is not currently possible to rename channels, or to delete messages or attachments. This will be possible in an upcoming version of Semaphor which will be available in a few weeks.
It is possible to delete channels. If you would like more information about that, or about team and channel management in general, please visit our article on how to Manage Teams and Channels on Semaphor.
If you are concerned about your data remaining on our servers, you can read our Semaphor whitepaper here: