Cannot install signed apk to device manually, got error "App not installed"

You may be using the android 5.0 or above device.

May be your development version is not uninstalled properly.

Try this,

Just go to the Settings --> Apps --> Click on your App. ---> In App info page at the action bar menu there will be an option called " Uninstall for All users " click that. Your app will be completely uninstalled and now you can try installing the new version with no issue. Hope this will help you

NOTE : This is for lollipop and above.Refer the screenshot attached.

For Current Updated Android Studio 2.3 users this answer is for you as hardly people use eclipse nowadays for Android development as Android studio has huge advancements.

So, Follow this way to create your Signed apk file.

1. Build > Generate Signed apk.
2. Create Keystore path.
3. Put Password, alias, key password.
4. Build type select accordingly(eg to release in playstore use release).
5. Signature Version select both V1 and V2 checkboxes.
6. Finsih.
7. Go to from explorer where you selected for the apk to store and you will see your .apk file name as app-release.apk use it.

selecting Signature Version v1 and v2 both solved the problem for me....try it