How can I remove creases from serigraph?  

Well,
my-mother-the-artist (serigraphy was her principal medium) told me that paper can be ironed to remove some creases (glad yours are not sharp folds).
I would work on the back (of course) and start with a mild iron. But let's wait for other advice. I personally usually do this by carefully bending the paper in the opposite direction, usually over a hard or semi-hard edge like the edge of a table. Wear clean cotton gloves to avoid transferring finger oil and other soils, and cover any surfaces you work on with clean paper or white cloth.

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I guess if the print inks are thick at all, there is a risk of cracking them by bending the paper. So ironing might be the safest approach.
An art dealer, gallery, or framing house might offer reliable advice. . .