Yes and no.
Once a contribution has been processed via a payroll, its designation as a Roth or “traditional” contribution cannot be changed.
Subject to administrative rules regarding the frequency with which you can change the amount of your 401(k) deferrals, you can change how FUTURE contributions are designated as Roth or “traditional”.
PLANNING TIP: If your plan allows for in-plan conversions, traditional, pre-tax plan balances can be changed to Roth account balances. This conversion is a taxable event and the tax liability must be paid with your annual tax return.