You didn’t mention how your backup was made, so the generic answer is: Usually with the psql tool.
Depending on what pg_dump was instructed to dump, the SQL file can have different sets of SQL commands.
For example, if you instruct pg_dump to dump a database using --clean and --schema-only, you can’t expect to be able to restore the database from that dump as there will be no SQL commands for COPYing (or INSERTing if --inserts is used ) the actual data in the tables. A dump like that will contain only DDL SQL commands, and will be able to recreate the schema but not the actual data.
A typical SQL dump is restored with psql:
psql (connection options here) database < yourbackup.sql
or alternatively from a psql session,
psql (connection options here) database
database=# \i /path/to/yourbackup.sql
In the case of backups made with pg_dump -Fc (“custom format”), which is not a plain SQL file but a compressed file, you need to use the pg_restore tool.
If you’re working on a unix-like, try this:
man psql
man pg_dump
man pg_restore
otherwise, take a look at the html docs. Good luck!