First, you need to manually download the new plugin package.
Then, you need to deactivate the current version of the plugin in the "Plugins > Installed Plugins".
Once you deactivate a plugin, you can delete it. Don't worry, your content will not be lost.
Now you can simply upload new zip file in the "Plugins > Add New" section.
Click on the "Upload Plugin" tab at the top and choose to upload the zip file.
Once it uploads, choose to activate the plugin.