My spouse is a US citizen and she is living outside the US for work. How I can change her residency

[Opus 17]

For what purpose?  The only address you need to put on your tax return is where you want to get tax-related mail.  It does not have to be the place you live.  As a US citizen, your wife is required to report and pay US tax on all her worldwide income, even if she is living outside the US. (She can claim a deduction or credit for taxes paid overseas on the same income.)

Her residency would only be important if she qualifies for the foreign earned income exclusion, which requires that she live overseas at least 330 days of the year. 

[Edited to add]

Her foreign residency could also be important for state income taxes, as mentioned below, but only if she has changed her domicile, which is her real permanent home and the place she would return to if the foreign assignment ended.

[TomD8]

If your wife is out of the country temporarily for work purposes, and her primary, main home (her domicile in tax terminology) is still in the U.S., then for tax purposes she is still a resident of her home state.  However, if she has moved to another country and that country is now her domicile, then in the Personal Info section of TT, at the bottom of the State of Residence drop-down window, select Foreign or U.S. Possession.