Some apps don't open, how can I fix it?
Generally, if apps quit right after they are opened, the two steps YOU can do are:


Go to the multitasking environment and quit the app(s) entirely (swipe the app window up and out of the screen), then try again
If 1. does not help, you can see about uninstalling and then re-installing it/them using iTunes on your computer.
IMPORTANT: Uninstalling and re-installing an app will also remove its own user data (e.g. documents or messages)


If both steps do not work to get the app(s) working again, you may want to contact the app developer(s) since there may have been some changes introduced in iOS 8.3, with which the app(s) cannot cope with and need updating as well.  Once an update is released addressing this issue, you will need to update those apps and then try again.