BuddyPress.org

You can’t; this is a result of the blog IDs being set by an auto-increment primary key integer in the MySQL database. You could do something like export the database, drop the table (wp_sites, or something), create the table again and re-import your data.

However I am not sure of the consequence on WordPress of this being done so not recommended unless you have time to check all the tables and update references where needed.