Jesus Himself defines for us what "overcoming" means in Revelation. That definition is found in Revelation 3:21.
"To him that overcometh, will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in His throne." Just how did Jesus "overcome" and then sit down on His Father's throne? He accomplished this by His crucifixion death. This was followed by His resurrection, after which He was enthroned at God's right hand in heaven. What to the world looked like Christ being defeated and "overcome" by physically dying was actually necessary for Jesus to "overcome" by resurrecting Himself from the grave. Apparent disaster was actually a victory, not only for Himself, but for any saints who are put to death for their faith in Him. "But thanks be to God who giveth us the victory through our Lord Jesus Christ". "We are more than conquerors through Him who loved us."
In those first-century days of persecution, for the believer who would try to "save his life" physically by compromising his faith under persecution, this person would "lose his life" by missing out on the AD 70 bodily resurrection. Likewise, whoever was willing to "lose his life" physically for Christ's sake and the gospel's during that period of persecution before the AD 70 bodily resurrection would thereby "save it" by being given the "crown of life" in that day. This doesn't have anything to do with losing one's salvation status as a child of God. Rather, it has to do with those dying before AD 70 being able to joyfully participate in that soon-coming bodily resurrection at Christ's return. Those saints who did not die before then would be left to wait until the next resurrection event in the distant future instead.
For all those in Revelation's 7 churches who were encouraged to "overcome", they were being given assurances that if they were "faithful unto death", they would be given the "crown of life" in that next, up-coming resurrection event when Christ returned.