Follow these steps
1) Backup your Database and Magento Root Directory.
2) Installed A clean installation of the version of Magento 2 you are using (important)
Use these links as reference to installing Magento 2 using composer:
3) Make sure clean installation is successful.
4) Delete the clean installation DB and Import backup DB
5) This step is for sites that already have content. Override some of the content of the clean installation with the content of the Magento Root Directory. Use the list below:
- (Magento Root Directory)/app/design/frontend/(Your themes)/ (Note:
only your themes, not the Magento theme unless you overrode anything)
- (Magento Root Directory)/pub/media/ (Note: Make sure this you
carefully select the right folders)
The list might change depending on your extensions and customization.
6) Make sure your site is in developer mode. https://devdocs.magento.com/guides/v2.3/config-guide/cli/config-cli-subcommands-mode.html
7) Finally, run
php bin/magento cache:flush && php bin/magento cache:clean
php bin/magento setup:upgrade
This should get you moved to a composer installation
Hope this helps.