[HELP] Decompile APK, fix, Recompile help

Hi, before all, sorry for my poor English.

I have buyed a new Android Watch; the SmartWatch U8. I have found many Android app to run it but nothing have great result.

After few test and search I have found the original APK for my watch, is run good but have some problems and I want improve it.

I have decompiled the APK (APK to ZIP, dex2jar and APK Tool), I have now some .jar files, the .manifest and few assets and others ressources.

Using Android Studio, I have tried to recompile, without modification of code. I have many, many errors.
Some errors is about depriciated, others is for unknown command or class.

I have tried to fix, but no chance, too many errors and my Java skill is realy low (I'm better under PHP). I have also try to copy only few codes using the error tools to known what is needed to add, but I don't known own the default structure was made (all class in same folder ?, or many folder for many class ? less class but with more actions ? not sure)

I have tried all ideas I have found, no valid result, I never build the app with success. I have made few Android App, with help of tutorials and are very basic (Web browser for my website, music player and scanner). I known the basic of Android coding and the new and old way to code them (Eclipse and Android Studio).

I search many solutions;
- [easy] Find another app with all (or the most) of needed change I want; hide the notification text but keep the icon when connected, more control of what is sending to watch (keep notice of call, but not make/answer call over the watch)
- [medium/hard] Find a good tutorial/guide for rebuild a Android app from decompiled APK.
- [hard] Build from scratch a Android app for my watch (with tutorial/guide)

The APK I try to edit: levelkro (dot) com/download/android/KHR1.3.22.apk (not available on Play Store)

Thank for your help:)