Before installing PostgreSQL, ensure to confirm with Gespage support that your PostgreSQL version is compatible with the database version.
Windows :
Go to the following page: EDB: Open-Source, Enterprise Postgres Database Management, and click the download logo for the desired version.
Once downloaded, click on open:
Once in the installer, click next until the installation completes
You have now installed PostgreSQL on your Windows server!
Linux :
Go to https://www.postgresql.org/download/ and choose your Linux distribution
Then, simply follow the listed steps 
You have now installed PostgreSQL on your Linux server!