SEOMATIC v1.1.45 install
To update plugins like SeoMatic, just download the latest (either clicking on the direct download link from the /admin/updates/ page in your admin area or from github).

Then you just then need to upload the new folder, overwriting the old one, to your server in the /craft/plugins directory (make sure you rename the download to seomatic as before, removing the '-master' that github adds).