Go to Sketchucation and click on the Pluginstore link at the top right of the page. Search for the extensions you want to update, download them and then use Install Extension under Window>Extension Manager to install them.
Better would be to install the Sketchucation Extension Store tool and use it to download and install extensions from Sketchucation. After you’ve used it to install extensions from there, it will keep track of extensions that need updating and you can batch update them. Much easier than manually downloading and installing each extension.
See this: