A level Physics lepton decays

Hi,
So basically I know that leptons can decay into other smaller leptons within their generation (their neutrinos) . But it says in my class notes that neutrinos can also decay/change into their corresponding lepton particle.
I don’t really get how this is correct because isn’t the whole point of a decay to become lighter and so more stable?

Could someone please tell me if this is just a mistake in my notes or if I just don’t get it lol.

Thanks (:

So leptons can decay and the neutrinos just oscillate?

Yes because the different flavours of electron have more mass so can lose that in decay as energy