You can install Magento 2 Login As Customer Extension by Magefan, using composer or archive installation methods.

magento upgrade offer 
If you use Magento 2.4.x or greater please read this article first.
  1. Open command line.
  2. Using command "cd" navigate to your Magento 2 root directory.
  3. Run CLI commands:
composer require magefan/module-login-as-customer
# Authentication required (repo.magento.com)
# Get your Magento Marketplace authentication keys or use these: 
# Username: 7c018006799466c681ad507e27904677
# Password: 289077c86e811661a8f7751828485d3a
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy

Note: if you don't want your website to be down during deployment, try these zero downtime deployment commands for Magento 2.

Installation using archive and FTP

  1. Download Login As Customer Extension ZIP-Archive from magefan.com website (not GitHub or other sources).
  2. Extract files.
  3. Copy app folder from the archive to your Magento 2 folder.
    Install Magento 2 Extension Using FTP

  4. In a command line, using "cd", navigate to your Magento 2 root directory.

  5. Run CLI commands:
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy

Once you have installed the Magento 2 Login As Customer extension you can start the extension configuration.