Removing hiss and other noise requires care and good judgement.
Audacity has a noise removal tool which can sometimes assist.
It is possible to remove hiss from a video by demuxing the video and audio streams. See below.
Software such as Virtualdub, VirtualdubMod and Avidemux can be used to extract the audio first, then the audio can be enhanced separately before being muxed again.
A VST plugin for Audacity, JS Noise Filtre is capable of producing decent results for all sorts of background noise.