
Hi,
First of all go and download the M159 technical manual from here;-
http://forums.watchuseek.com/f365/seiko ... 99038.html
This details the exact procedure for replacing the crystal in this watch- the only problem is the plastic support gasket which, if you didn't get a new one with your crystal, has to be salvaged to refit the crystal and it is all too easy to ruin when extracting the old crystal.