Hello ,
Step 16 in the article link has you remove the old install of Postgres.

https://community.qlik.com/t5/Official-Support-Articles/Upgrading-and-unbundling-the-Qlik-Sense-Repo...

16. Uninstall the old Qlik Sense Repository Database service.

Note : This step is required. Failing to remove the old service will lead the upgrade or patching issues.