Since your Switch's BootROM is already at version K.12.20 (Note: minimum BootROM version the switch should run to be able to upgrade directly to any software branch of K.15 is BootROM version K.12.17 so you're pretty OK with that) I suggest you a safe approach to reach K.16.02.0025 (June 2018): first go to through an interim release on K.15 such as (any of the K.15 will be valid...so I'll take the latest) the K.15.18.0019 (April 2018), this one uses BootROM K.15.30...once your switch is running on K.15.18.0019 with the BootROM at K.15.30 you can do the final update to K.16.02.0025 which still continue to use BootROM K.15.30 too (it isn't changed).
Basically any update to Software Version K.15.xx.yyyy (xx = 04, 06, 07 up to latest 18 | yyyy built number changes within each branch revision xx) will update your BootROM too from actual K.12.20 to something K.15.zz (zz is actually 30 but, with lower xx versions had lower values like, as example, 19)...since you have the BootROM K.12.20 (newer than K.12.17) you can safely go to latest K.15.18 and have your BootROM updated to K.15.30, this will not change when you then update to K.16.02.0025.
Another approach - supported too - would be: bypass the interim update to K.15 and go directly to K.16.02.0025, this is possible because BootROM, as written, didn't change its major version K.15.zz between first K.15.xx release and latest K.16.xx release.
So you have two possible approaches: one with two reloads and the other with just one reload...it all depends on how long your switch can be down (for rebooting/reloading).
Use show flash and show version to check what your switch currently has on primary and secondary partitions plus BootROM.
References here.
