Use the same procedure as when you installed the theme for the first time.
Everything you did on your Dashboard won’t get overwriten, the files you’ve modified, however, will be replaced with the new files. I recommend that you save them in separate folders so you’ll have them at hand to reapply the changes.
I use this handy plugin for auto updates of all plugins and themes –
It makes a backup of the existing plugin/theme as well in the media folder so you can always revert back or refer to the earlier version. If you made any changes to the source files (instead of child theme), you will need to redo them of course no matter what method you use for updating.