The JDK configuration for JDeveloper is defined in the jdev.conf which located in your Jdeveloper folder JDeveloper\jdeveloper\jdev\bin.
Open the file and find the item: SetJavaHome. Change its value to your new JDK folder.
In the below example I switch JDK from JDK 7 to JDK 6 by commenting out JDK 7 and adding a new item for JDK 6.
#SetJavaHome C:\Java\jdk1.7.0_06 SetJavaHome C:\Program Files\Java\jdk1.6.0_45
After saving the changes restart JDeveloper and it will use the new JDK set in jdev,conf.
You can check JDK used in JDeveloper from JDeveloper. From JDeveloper goes to Help->About. Then select the tab Properties and will find java.home properties now is set to the JDK specified in jdev.conf.