how to unglue

I assume you mean solvent cement when you say plastic cement.  That stuff actually welds the plastic together, so it is pretty hard to undo if it is really set.

That is one nice thing about CA.  You can buy CA debonder, and apply it weeks after the glue has set and it will dissolve the bond.

your a life saver CC ,don't know how I did it , but I glued 2 mast's backwards , out of 4 that really looked weird . it's all better now , cheer's mate

steve5

If the cement has not dried completely, I grab the part with a needle nose pliers and horse it off.  Hopefully, I can then reglue it properly later.  If the glue is totally set, maybe,  depending on what kind it is, you can put more glue or sovent on it, and then proceed as above.