After you have installed JComments, you have got an ability to change and update JComments templates. There are two ways for doing this, but the second one is preferable – it makes installation cleaner as it deletes folders and files which will not be necessary anymore.
But before you start updating, you will need to get an updated template. To do this, go to the template page and check if its version is newer than the one you are currently using. You may also look at changelog to understand if you need an update at the moment. If there is an update package and you want to do it, access your downloads area at www.roundtheme.com/downloads and get it.
Now its time for actual installation.
Way 1. Normal Installation
To install the template normally, go to your Extension Manager in the Administration Panel and browse for the package you downloaded. After you have selected it, click Upload & Install.
Way 2. Manual replacement
This way is a bit more sophisticated. Access the folder of your website called \components\com_jcomments\tpl\[yourtemplatename] with SSH, FTP client or cPanel. Make a back-up copy of the contents of the folder. Now replace all the files in it with the contents of the package you downloaded and you are good to go.