First, go to https://codecanyon.net/downloads#item-17870313 to download the latest version of Lastform:
To update the plugin, the easiest way is to install Easy Theme and Plugin Upgrades plugin. Then, you can simply upload it again.
But, if you don't want to install another plugin, you can deactivate Lastform, delete it and then upload the new version. Don't worry, you won't loose anything.