You enter your JDK path and Oracle SQL Developer uses it for running in Java JVM. Now what if you want to change it? It is easy. Open [sqldeveloper_installation]\sqldeveloper\bin\sqldeveloper.conf (on my machine the path is C:\tools\sqldeveloper\sqldeveloper\bin\sqldeveloper.conf) and modify it as following:
That is, comment out or remove the old path to Java SDK and place the new path. Now restart Oracle SQL Developer and verify that it uses the new path by going "Help->About" and selecting "Properties" tab:
