How to install old PHP composer v1

Composer V2 was release this week https://blog.packagist.com/composer-2-0-is-now-available/ but your project may not supporting it yet.

If you want to rollback to V1 one, you can use the following:

composer self-update --1                                                

You can find available versions on the composer download page https://getcomposer.org/download/