WordPress provides a method to upload any available WordPress plugins.
You can also resolve many upload or upgrade issues with plugins by using the WordPress Upload Plugin feature.
This feature is located on the top of the Appearance > Plugins > Add New section in WordPress.
Note that the plugin file must be a .zip file
NOTE 1: You will want to back up your entire WordPress site before any upgrades or uploads. You want to have a complete copy of your site so that you can always revert back if you have issues. This is a good idea whenever upgrading or uploading any plugins.
NOTE 2: If you are Upgrading WishList Member (you have a copy of WishList Member already installed) please be sure you have deleted the existing WishList Member plugin from your site before trying to re-upload using the WordPress Uploader. All WishList Member settings should be retained as they are stored in the WordPress database but be sure to create a full back up of your site before any plugin deletion/deactivation/upload.