If it is square cut the end which is as you would with a sliced loaf and pull slices out ( you can cut the corners a little to make it easier. Try to avoid letting in too much air or it will spoil.
If it is round the centre will keep fresh if you are careful.
I have had 2 wobblers. My first one was diagnosed after rearing up over backwards as a yearling colt. He was down 18 hours and eventually got up. He was such a beautiful and well bred horse that we decided to keep him and he eventually died at 21 years. We used him on mares and he produced some...