I heard downgrade Nintendo switch is possible, how can I do it?
Downgrading is not supported at all on the original firmware, so I assume you're asking about a currently modded device, or a device you plan to mod before downgrading.
One feature of Nintendo Switch hardware directly related to upgrades is called "the fuses". A fuse is a special region of internal memory which can only be written to (or "burned") one time, and once burned, it stays that way. Small upgrades don't burn fuses, but important upgrades do, and 6.2.0 was exactly one of these important upgrades. The number of burned fuses is checked by the bootloader every time your Switch starts up or wakes up, and the device will reset on a mismatch.
So no, there's no way for you to roll back to an earlier original firmware.