Postgress configuration files are located at:
/etc/postgresql/##/main/postgresql.conf
Use your favorite code editor and navigate to the CONNECTIONS AND AUTHENTICATION this will reveal at which port the Postgress database is listening.
Connect to you desired version using the -p flag. For me Postgres 11 is using port 5433 and Postgres 12 is using 5432.
psql -h 5433 postgress
Check your version using
SHOW version();
Expected output
version
----------------------------------------------------------------------------------------------------------------------------
PostgreSQL 11.9 (Ubuntu 11.9-1.pgdg20.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 9.3.0-10ubuntu2) 9.3.0, 64-bit
(1 row)