You can use pg_restore to push a dump file from a local database to an Heroku database.
If you go to the Herokue Postgres page for the database they give connection settings for a number of utilities, one of them being pg_restore. The command will look similar to:
PGPASSWORD=jyb4fhiwyeioghfwc7hw pg_restore --verbose --clean --no-acl --no-owner -h ec2-23-254-171-15.eu-west-1.compute.amazonaws.com -U hqfy134fblqwheb -d sdjhufhqwuf -p 5432 [YOUR_DATA_FILE]
Just substitute in the name of your local file. If the Heroku database is completely empty you can omit the -clean option and save some time.