If you have installed b2evolution tables before and wish to start anew, you must delete the b2evolution tables before you can start a new installation. b2evolution can delete its own tables for you, but for obvious security reasons, this feature is disabled by default.
To enable it, please go to the /conf/_basic_config.php file and change:
$allow_evodb_reset = 0;
to
$allow_evodb_reset = 1;
Then reload this page and a reset option will appear.