Oracle SQL Developer is a popular tool to connect to Oracle database and perform SQL queries (and not only). When you start SQL Developer for the first time it asks you where your Java SDK is.
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: