To change taxes, go to Employee Management > Employee Pay > Tax Information.
Change the exemptions or add additional taxes where desired. If you are missing a tax jurisdiction you think should be available, double check the Resident Location (home address) and Work Location.
Make sure you have the employee fill out the appropriate government documentation related to the change in withholding status so that you have the proper records. The IRS form is the W-4, and each state that has state withholding will have a form as well. Generally, these forms are easily found using a search engine (e.g. for California, search "California state withholding form for employees").