There easy to replace you just need a c spanner and you can get larger or smaller or the same size main chainrings if you want to gear it higher or smaller from Martin at the ebike shop
Hi Simo
I've taken mine off there's a couple of screws hidden behind the main chain wheel you can just get to it with screwdriver pliers you may have to wreck the
chain guard to remove it tho,
it seems strange that the difference jumped from 43cm to 50 cm I think you should get the frame changed for a smaller one as you not happy with and were misled a bit.