Windows :
Go to the following page: EDB: Open-Source, Enterprise Postgres Database Management click on the download logo in row 15.15 of the Windows column
Once downloaded, click on open:
Once in the installer, click next until the installation completes
You now have PostgreSQL version 15.15 on your Windows server!
Linux :
Go to https://www.postgresql.org/download/ and choose your Linux distribution
Then, simply follow the listed steps 
You now have PostgreSQL version 15.15 on your Linux server!