How do you replace the adaptor jack on a Casio WK-1200 keyboard ? is it easy ?

Unfortunately the adapter jacks are a bit of a weak point on many keyboards but it is quite seldom the jack socket itself that is at fault.
Often the sockets are soldered directly on to the printed circuit board and are prone to either one of the soldered legs breaking, or even the circuit board itself cracking around the jack socket. 
Whilst it is generally possible, with care, to resolder a broken leg on the socket if the actual circuit board has cracked the whole board will have to be replaced.
If the actual jack socket itself is damaged it should be possible to snip through the legs and remove it before soldering a new one on to the stumps of the old legs. this is likely to prove easier than trying to solder directly onto the circuit board, unless this is something that you are experienced at doing.

Apparently you can purchase a full service manual for the Casio WK-1200 here for a modest price but I have not used this source myself so cannot vouch for how good it might be.http://www.tradebit.com/filedetail.php/2084335-repair-manual-casio-wk-1200-electronic-keyboard