I've been using 6u10 beta for a while and keeping 6u3 as my main JDK. It's also the default JDK for NetBeans but now that update 10 of Mustang is out of beta, it's time for me to ditch 6u3 and swtich to 6u10 final. This is fairly straight forward. Here's what I did:

Install JDK 6u10 (it'll find 6u10b and offer to remove it)
Start NetBeans, add the new 6u10 and remove the (now missing) 6u10b java platforms. Do this from the Tools > Java Platforms menu
To change the default JDK (which is the one running NetBeans itself)
1. edit /path/to/NetBeans/etc/netbeans.conf (you can do this in NB if you like)
2. change the netbeans_jdkhome property to be the path to your 6u10. On my Windows machine, this is "D:\Java\jdk1.6.0_10"
3. Restart NetBeans
(optional) Uninstall your other jdk6 and remove it from NetBeans' list.

More tips for netbeans.conf can be found on the NetBeans Community Wiki. My policy is to keep around only the latest stable update of each JDK, so I still have 1.4.2_17 and 1.5.0_15, in addition to (now) 1.6.0_10. Mainly this is because I still use 1.4.2 for work and 1.5.0 for testing. But there's no point in keeping two updates of the same version because their APIs are the same. Same goes for the javadoc: it shouldn't have changed between 1.6.0 and 1.6.0_10 so I haven't bothered to check it or to download a newer copy. I also keep (zipped) javadocs for 1.4.2 and 1.5.0.