How do United Pentecostals and other Oneness groups interpret Hebrews 2:9? But we see him who for a little while was made lower than the angels,
  namely Jesus, crowned with glory and honor because of the suffering of
  death, so that by the grace of God he might taste death for everyone.
  (Hebrews 2:9 ESV)


If there is no distinction between the Father and the Son, as Oneness adherents believe, how could God who "upholds the universe by the word of his power" (Hebrews 1:3 ESV) die without the universe falling apart?
United Pentecostals do not deny that there is a Father and there is a Son.  They believe the Father and Son are one Spirit.  They believe that there is a distinction between Father and Son.  The Son is God in the Flesh.  The Father is God not in the flesh.  The United Pentecostals point out that in Scripture, the term "Son" is always used in reference to God in the flesh. If you would like to read more of what the UPC doctrine is, I suggest "The Oneness of God," by David Bernard. (I am not UPC, thus, I hesitate to go any further specifically regarding their doctrine, lest I misrepresent them).  

As a Oneness adherent, I would continue thus in regard to this verse and your question: 

Flesh is mortal, spirit is not. Flesh can suffer, flesh can die, the spirit experiences the suffering of the body, but the spirit does not die; Thus, When Christ died on the cross, God did not die.    Note:


  Luke 23
  46 Then Jesus, calling out with a loud voice, said, "Father, into your hands I commit my spirit!" And having said this he breathed his last (NKJV).  /"gave up the ghost" (KJV).


So, God did come in the flesh, and did experienced death in the Body he took on; He suffered, and in the sense that any man dies, he died.  But God (Spirit) did not die.