Dar, is the ceiling dirty or broken tiles? If they are old fibrous tiles, then it's likely they contain asbestos. In which case, it's best to leave them in tact and cover. They make really beautiful ceiling tiles that flush mount.
If in good condition...paint. What seems to be the problem? What do you want to do?
How to Replace Ceiling Tiles with Drywall | how-tos | DIY
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