Dropping a co-convener from the list? I propose a session every year in an International conference in USA with 3 more co-conveners and maximum 4 are allowed including me. I am thinking to drop one of them in this year and assign a new member in the list. Although I manage everything from writing the proposal until chairing the session, but still I feel awkward to drop anyone from this list. But I have also no other option, as I need to add a new member. Now following two questions are striking me:
1- Because the title and proposal are same as previous years, is it
ethical to drop one who was already a co-convener of the same
session since last 5 years?
2- How to inform the one that you won't
be in the list this year?
Conveners change. After five years, surely change is expected. Dropping one of your co-conveners is a little suspect though, it suggests an individual is at fault. I recommend dropping several co-conveners and having a more radical re-shuffle.

Because the title and proposal are same as previous years, is it ethical to drop one who was already a co-convener of the same session since last 5 years?

Assuming you have no agreement, then, yes, it is ethical to drop and add co-conveners. Journals and conferences typically work this way.

How to inform the one that you won't be in the list this year?

That's been partly answered elsewhere on this site and is partly dependent on your individual circumstances.