Sounds like a major PIA! Not an easy job, requires specialty tools and taking down the headliner amongst other things.
I replaced my rod antenna with a shark, one off amazon supposed to be specifically designed for the Leaf and it was probably as much of a PIA as the youtube posted here. Long story short the fin did NOT fit over my existing body(the part the rod screws into) and required me grinding my body narrower, using a hack saw and modifying the new fin, NOT an easy job and took hours and hours to complete, I would NOT do this again. Now if you could find a properly sized fin(larger) it might have been easy with the included 3M tape, simply unscrew the rod, screw in the wire, use included 3M tape to attach it to your roof but unfortunately for me, this was not the case. I left my review on Amazon saying this fin did not fit Leafs, at least '12 and '13 models like it was supposed to, I don't have access to a newer Leaf to see if they might have a smaller body.....
Note I didn't replace my rod just to have a fin, the body threads ended up cracking making the rod not fit securely after driving through a car wash, the threads were strong enough to hold the wire from the fin but not the rod which due to wind blowing over it had more leverage on the threads. I lost a few further away FM stations as the fin isn't as good as the rod and while I never listen to AM, I also lost all but the strongest AM stations.
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a