Scripture is silent regarding when Peter died. It also does not directly tell us the precise manner of how it occurred. Only one Biblical verse hints at the possible circumstances of his demise.
One can be assured, however, that when Peter perished he had the faith and conviction to do so as a Christian. This he was not able to do when Christ was alive (see Matthew 26). Most of what is stated regarding the end of his life is based on tradition and speculation. The belief of many Biblical commentaries, and Christians in general, is that he was put to death through being crucified, upside-down, in Rome.
"Among many other saints, the blessed apostle Peter was condemned to death, and crucified, as some do write, at Rome . . ." (Foxe's Book of Martyrs).
"The time and manner of his martyrdom are less certain. According to the early writers, he suffered at or about the same time with Paul, and in the Neronian persecution, A.D. 67, 68. All agree that he was crucified. Origen says that Peter felt himself to be unworthy to be put to death in the same manner as his Master . . ." (Smith's Bible Dictionary).
Dying as a martyr
Jesus, shortly after his resurrection, told the apostle he would die as a martyr. Peter acknowledged this prophecy in the last epistle he wrote sometime between 65 and 66 A.D.
Truly, truly I (Jesus) say to you (Peter), since you were young, you have dressed yourself and walked wherever you have desired; but when you are old, you shall stretch out your hands, and another shall dress you and bring you where you do not desire to go.
Now He said this to signify by what death he would glorify God. And after saying this, He said to him, "Follow Me" (John 21:18 - 19, HBFV throughout).
Knowing that shortly the putting off of my tabernacle will come, even as our Lord Jesus Christ has signified to me (2Peter 1:14).
How was he killed?
John's verses suggest, but do not clearly state, that the apostle would meet his end through crucifixion. The phrase "stretch out your hands" does not necessarily mean his death was by a cross. The phrase could also mean that the he would have his outstretched arms bound, requiring someone else dress him before being led to his execution.
Even if we assume that Peter was crucified, note that John 21 says nothing regarding it occurring upside-down.
Some religious people and groups believe Peter met his end in Rome since, they believe, he resided in and preached there for a long stretch of time during his ministry. Scripture, however, offers evidence to the contrary. His last recorded book in the Bible, written between 65 and 66 A.D., does not refer to Rome. Since tradition states he likely was martyred around 67 or 68 A.D., any time spent in Rome was likely quite short.
How did the apostle Peter ultimately die? We are on safe Biblical ground in stating he perished as a martyr. Tradition states that he perished under Roman Emperor Nero in or just before 68 A.D. It is unclear whether he was killed through crucifixion, and only certain traditions state that he perished while upside-down. It is entirely possible his end did not happen in the city, and through the means, most people believe.