If it’s not very bad, the best way is to do it manually, with volume automation. Just pull the volume down when there are breath noises. This keeps the sound natural and doesn’t create any artefacts. It also lowers noise.
See the envelope tool to discover how to do it:
http://manual.audacityteam.org/man/envelope_tool.html
As a sound editor, you really need to know this tool anyway. It’s probably the most used tool in audio editing, before eq, compression or whatever.
If it’s bad, consider posting a sample here. For a how-to on uploading samples to the forum, have a look here:
https://forum.audacityteam.org/t/how-to-attach-files-to-forum-posts/24026/1