The extension must be installed via composer. To proceed, run these commands in your terminal:

composer require shoppingfeed/magento2-manager
php bin/magento module:enable ShoppingFeed_Manager
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy

To update the extension to the latest available version (depending on your composer.json), run these commands in your terminal:

composer update shoppingfeed/magento2-manager --with-dependencies
php bin/magento setup:di:compile
php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy

Sometimes, running composer update does not actually update the module to the desired version, for example because it does not match the version constraints specified in your composer.json (run this command to know the exact reason why: composer prohibits shoppingfeed/magento2-manager).

When this happens, simply adapt your composer.json to point to the new version you want to install, before re-running the update commands (and if that would still not be sufficient, do not hesitate to post an issue so that we can have a look at it).

You may want to enable the maintenance mode when installing or updating the module, especially when working on a production website. To do so, run the two commands below before and after running the other setup commands:

php bin/magento maintenance:enable
# Other setup commands
php bin/magento maintenance:disable