Yoke Foon,
Yes, you can apply the JRebel update in MyEclipse Configuration Center even if you are currently on ME 10.1. Your JRebel for MyEclipse would be upgraded to 5.x.
Here are the steps –
1. Select the software update “JRebel for MyEclipse” to be updated
2. Click on [Apply 1 change] under *Pending Changes* section
3. Follow the wizard till completion, when prompted choose to restart the IDE
4. After installation, if you have not activated your license, you would be prompted to activate your jrebel license, please do the needful
5. To verify the version of JRebel, you can do the following, open MyEclipse Blue IDE, from menu options click on Help > About MyEclipse Blue Edition > Installation Details. Select the Plug-ins tab and look for “JRebel Eclipse Plug-in” under *Plug-in Name*, you can see the version updated to 5.0.0

Hope this helps.