run composer require sveaekonomi/magento2-checkout && composer install to install files. Then run bin/magento setup:upgrade && bin/magento setup:di:compile && bin/magento cache:flush from your base folder to install database scripts and recompile the dependency injections and clear cache.

After that, head to administration -> Stores -> Configuration -> Payment methods -> Svea Ekonomi Checkout, to configure your new payment method.

To upgrade run composer update sveaekonomi/magento2-checkout --with-dependencies. Then run bin/magento setup:upgrade && bin/magento setup:di:compile && bin/magento cache:flush from your base folder