Hi there, if you download the new version of the theme and unzip it. Extract the theme files and you can then upload them via an ftp program like filezila or via your hosting companies interface, like c-panel etc. When you upload the files, they will rewrite the existing ones with the new theme files.

A guide on how to do this can be found here – http://www.amazewp.com/how-install-update-wordpress-theme-manually/