Assuming that the plugin was supplied by someone who knows how the release was made (java versions, etc etc), this is really easy.
1) close any JGrass session you could have open
2) get the new plugin ( ex. download the plugin from the jgrass site) and put it into the same folder in which the jgrass installation folder is.
3) extract the plugin if it was compressed
4) enter in the jgrass installation: grass->plugins and remove the plugin you are going to substitute.
5) copy the downloaded plugin into the plugins folder
6) restart JGrass and as usual... enjoy!
PS: this obviously works for any plugin, therefore also eclipse plugins (if you want to add a tetris plugin to JGrass, you could judt do so) and udig plugins