The Employer's Legal Resource: REMINDER - New Form I-9s Must Now Be Used
Employers are now required to use the new Form I-9. You will know you are using the correct form if the date at the bottom left-hand corner of each page bears the date 11/14/2016. If it does not, you are using an outdated form.
Also, the new version has the instructions on a completely separate form, also bearing the date 11/14/2016 on the lower left-hand corner. USCIS's news release briefly describing the changes can be found here. Interestingly, USCIS has not (yet) updated its Handbook for Employers which was last updated in 2013.
You can find all of these documents on the USCIS's website.