when  i apply a patch and compile it what changes should i make on server folder ??''c:/server''

It fully depends on what you edited lol. In almost all cases, l2jserver.jar is the only thing to replace.

when  i apply a patch and compile it what changes should i make on server folder ??''c:/server''

in case you touched datapack,then you will have to replace either the whole datapack or the lines which you edited.

 

in case you touched core,l2jserver.jar