Note : Its always recommended to backup your Website, Theme, Database before updating. You can use this updraftplus plugin to have regular backups of your Website.
Updating the theme is very easy.
To update theme, you can use this Envato Market plugin. This is created by Themeforest itself, and there are just simple steps to follow once and you can update easily always in future.
After updating the theme, just go to Appearance->plugins to update all plugins to their latest versions.