Installation of Yaplex.SEO can be done the same way as any other modules for Orchard CMS. First you need to download latest version of the module, you can use the following link from Orchard Gallery https://gallery.orchardproject.net/Packages/Yaplex.SEO
Navigate to Modules -> Installed and click on
Install a module from your computer.
Choose file which you just downloaded from Orchard Gallery and press
You will see successful message. Just press "Ok" button to enable module.
You may navigate to Modules section to double check if module was installed and activated.