Before explaining how to updates plugins you need to know that there are 2 kinds of plugins: Premium Plugins that we give for free and Free Plugins that you will find in wordpress repository plugins.
If you need to update a free plugin simply click in your Dashbord > Plugins > Plugin name > Update Now. WordPress will download automatically the plugin update.
Note: Please make a backup of the plugin before update it so , if you are facing problems, you can restore the old version. However our advice is to never update a plugin if you’re not sure about the compatibility with the current version of your theme.
For update a premium plugin it is required to update also the theme. For this reason the steps are:
Our advice is to never upgrade a plugin without knowing if the theme is also compatible with it. The theme will automatically re-download the plugins that you have just deleted in the new version.
More information about the premium plugins here: Visual Composer and Revolution Slider.
Note: If you have modified the plugin core your changes will be lost.