It depends on how soon you would like to see the change in your withholdings. To change your withholdings sooner rather than later, you may use the 2017 version. It is the only version currently available on the IRS website. According to the IRS, as of January 2nd, they are working to update tax forms to reflect Public Law 115-97, Tax Cuts and Jobs Act. For more information, please visit their website at this link:  IRS Legislative Impact on Tax Forms  If you need to change your withholdings again after new tax forms are available, you may submit a new Form W-4 to your employer.