Recommended method of updating firmware is using Z-wave JS UI (Aeotec's recommendation)
If updating from an older version than 7.18.8, make sure to first flash 7.18.8 before flashing a newer version, if not this could softbirck your stick (stick will be stuck in bootloader mode). If this is the case, refer to this post to unbrick your stick: https://community.home-assistant.io/t/did-i-just-brick-my-z-stick-before-even-using-it/734837/4
If your stick is already running 7.18.8 or newer, it is safe to flash any other newer version (i.e from 7.18.8 directly to 7.21.5)