My encryption password is too simple and was wondering how I could change it without having to reset the whole phone?
I know there is an app for Android 4, but what would be the procedure on an Android 5 device?
Device: Nexus 5
OS: Android 5.0
There currently is no such app that does this on Android 5.0 so you should back up your data and wipe the phone and start again with a new password.
Your encryption password is the same as the pattern/PIN/password you use to lock your screen. So simply go to Settings > Security > Screen lock and change it. Note that when you do this Android will ask you whether to also enter the pattern/PIN/password when you start the device in order to make your encrypted device's data unreadable without it.
In Android versions before Lollipop, it was good practice to change your encryption PIN/password to be different from your unlock PIN/password and make it longer/stronger as the encryption key was fully based on this PIN/password. So if you kept it short and easy (which most people do in order to quickly unlock the phone) your device could be brute-forced seconds/minutes (by making an image of your device and circumventing the 5x wrong password time-outs).
Since Android 5.0, the encryption key is only partly based on your pattern/PIN/password, the other part is based on a Trusted Execution Environment’s (TEE) key stored on your device in a separate hardware component (at least for high-end devices, including the Nexus 5 AFAIK), making the previous brute-force method impossible. Still, only using a 4-digit PIN can be brute-forced within a minimum of 17 hours (including the 30 second timeout after 5 incorrect tries)! Using a 6-digit PIN will take nearly 70 days. On the other hand, a 5 character password (using only the characters from the alphabet) will take 825 days minimum :).
So this info will tell you whether your current 'too simple' password is indeed too simple. If this is the case then change it as described above.