The most safe way to work on a Krag rear-sight is to remove the sight from the rifle.

'Whig' has advised you appropriately. With proper screw-driver fit and adequate pressure to keep the bit in the screw-slot, there should be no harm.

Your front screw appears to be a replacement. It is sitting 'high' and has a bevel on the head. It may be a reproduction. It is likely a model-1896 (sight) front-screw, some of which have a high head.

Both screws, for the model 1902 sight, are identical and should sit flush.

It is not unusual for Krags to sometimes have a wrong sight-screw. (Replaced in field or by WW1 period Armorer)?

Attached photos to assist you:
1. Your high screw-head.
2. Correct 1902 screw appearance.
3. A variation of the 1902 sight with attaching screws, dismounted.
4. A Krag 1902 sight taken apart to replace a broken knob.
5. A model 1901 base held safely in a vise to aid reassembly. (A 1902 base and leaf can be held similarly, to compress spring, and aid pin removal and replacement).

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