This will help you to change any running php version into ubuntu.

Fix PHP Multiple Version - How to Switch or move back to Installed PHP Version

1. Call this command
      updatedb
2. Locate php bin file
      locate bin/php 
3. check directories
      ls -l /usr/bin/php
4. Remove current php linking into bin dirctory
      rm /usr/bin/php
5. Create link for php version you want to use it (here is need to back to 7.4 - it must be installed)
      ln -s /usr/bin/php7.4 /usr/bin/php 
6. Check php version
      php -v
      
      
Boom Its Done.