This page of our trophy guide to Sniper Elite 4 contains a detailed description of Channel Changer trophy. This achievement requires you to take control of the enemy held radio and capture it with just seconds remaining.
This achievement is obtainable only in the multiplayer game mode, in which you capture airborne radio stations. Two teams struggle to capture them - in order to do that, players need to get very close to the station. The meter in the upper part of the screen depicts the progress of the capture. When another team manages to get to the radio, it's they who are now capturing.
To unlock this achievement, you have to kill the enemy shortly before the capture meter reaches 90%. The thing is that you (or someone from your team) need to instantly intercept the capturing process. You can walk your way around this by starting a 1v1 game with a friend, and doing this achievement. Otherwise, in bigger games, you ought to take a vantage point near the radio and wait for the right moment to push, which isn't easy.