The key combination varies slightly depending on whether your J7 has a physical Home button (J700 series) or the newer capacitive buttons (J710 series). Below are the two proven methods.
For J7 Models Without a Physical Home Button (J7 Duo, J7 Prime 2) Power down your smartphone. Press and hold buttons together. Release them when the warning screen shows up. Press Volume Up to continue. Understanding the Warning Screen j7 download mode
If your J7 still boots into Android but you want to force Download Mode: The key combination varies slightly depending on whether
(often referred to as Odin Mode ) is a special state used by Samsung devices. It is distinct from the standard Android Operating System and distinct from Recovery Mode. While Recovery Mode allows you to wipe cache, perform factory resets, and apply updates from external storage, Download Mode is used for low-level system flashing. Press and hold buttons together