The easiest way is to carefully use a cell scraper to lift the spheroid of the plate cover and transfer it to a well of 24-well plate or so containing medium. Then add 1/10 volume of 37% formaldehyde with gentle swirling while adding, and fix at room temp for 30 mins. Or you could just premix the formaldehyde-medium just before adding the spheroid so you don't have to do too much swirling. Then remove the single cells that separate out.
100% Methanol at -20C can also be used if your antigen can take it.
Thank you for your suggestion. I want to do Prussian blue staining on these spheroids. Do you have any idea about it? would you please share with me if you have any article or protocol?