I am hoping we will get some Annona experts on to look at your photo and verify this is a Rollinia. Something tells me there was discussion of a yellow Atemoya in the past, but i couldn't find the link. If it is a Rollinia, it may be close to ripe since they ripen really fast once picked.....like in hours to a day. If it is an Atemoya it may take a few days to ripen fully. Is it really firm still? Hard like a rock, or can you press on the surface and it has just a slight give to the flesh?