You don't.

When you want to reset data, you don't delete the old data - You just don't load it. This either means changing the name of the datastore, or changing how you generate the keys: If you're generating the keys based such as "user_" .. Player.UserId, then resetting all player data is simple as changing user_ to something else.

This is an unfortunate limitation in how Roblox handles datastores, however there is not a better way to manage this as Roblox only exposes an in-game API and expects persistence for the foreseeable future; They also expect you to use a testing database for testing, or to not enable Studio API access.