Why white key (F4) and black (F#4) of the upright piano sticking between it'self. How fix it DIY.

The first thing to do is to run a thin blade, even a credit card, between the two keys to knock whatever crud that might be jamming the keys beneath them. If that does not work, you will have to open the piano, and then run it along the back of the keys.
If that does not work, you have to look for interference higher up in the action.

Issue: The keys of the upright piano, specifically the white key (F4) and the adjacent black key (F#4), are jammed together.

Script:

1 Key Removal:
Carefully extract the black key (F#4) and the white key (F4) to facilitate the repair process.

Adjustment Procedure:
2 Utilize a screwdriver to manipulate the keys on 'the balance rail key pin'.
For the black key (F#4), apply gentle lateral pressure towards the right side.
If required, shift 'the balance rail key pin' marginally to the left for the white key (F4).
Further Actions:

3 In case the keys remain stuck, consider refining the plastic edge of the white key (F4) that interfaces with the black key (F#4) using a file.
In case the keys remain stuck, consider refining the plastic edge of the white key (F4) that interfaces with the black key (F#4) using a file.
4 Alternatively, if the issue persists, try pushing the black key (F#4) away from itself on the balance rail key pin.

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