Android
Change Screen Lock PIN, Password or Pattern auf Android
Switching your screen lock method on Android does not require removing the old lock first. The system simply asks you to confirm your current PIN, password or pattern before letting you set a new one. This confirmation step exists to prevent someone else from changing your lock screen if they briefly get hold of an unlocked phone.
The setting lives inside the device security menu, not inside a separate lock screen app. Go to Tap on Settings ➔ Tap on Security & privacy ➔ Tap on Device unlock ➔ Tap on Screen lock, then Select Pattern, PIN or Password and follow the instructions on screen. After you confirm your current unlock method, you pick the new type and set it up the same way you did the first time.
When this does not help
If you cannot get past the confirmation step because you forgot your current PIN, password or pattern, this menu will not help you. In that case you need your device recovery options or a factory reset, since the security settings are locked behind the very credential you are trying to replace.
Switching between lock types (for example from pattern to PIN) can also affect connected features such as secure folder access or app lock, since some of those rely on the same credential. Check those areas afterward if they suddenly ask you to set them up again.