File and widen the finger holes so that the pitch will tuned firmly.
One note at a time, check the tuner and widen the holes.
You can make the low notes higher, but you can’t make the high notes lower, so let’s be patient and careful .
Even if you get it right once, the pitch may change due to temperature or your blowing and etc., so you should spend several weeks , gradually widen holes and perfecting it .