When reinstalling CiviCRM, it will not allow connection to a database with preexisting data
You'll need to use mysql or phpmyadmin or equivalent to run sql/civicrm_drop.mysql. You may also need to manually remove the civicrm_install_canary table.

What it's technically complaining about is that you still have data in the civicrm_contact table.

UPDATE for more instructions using phpmyadmin: On the Import tab browse for the sql/civicrm_drop.mysql file and then click the Go button. You won't be able to browse on the server, so you'll need a copy of the civi download unzipped on your machine to browse for it.

To drop civicrm_install_canary, go to the SQL tab and enter DROP TABLE civicrm_install_canary and click Go.