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The battery is the problem, only if the battery indicator has altogether disappeared (or, is flashing completely Off, and On, at times.)
On the lower front of the rangefinder, you will find a round cover with a recess. Using thumb and fingers, rotate that round cover in the direction of the open-close arrows.
It may not open easily, it is waterproofed with a seal, and may be hard to do. If it proves impossible, then you will have to take it to a shop.
Insert the new battery with the + pole facing inside the device.
The new battery should be like the old, a CR2 Lithium disc battery.
To close the battery cover over the new battery, align the white dot with the Open/Close indicator, and turn the cover following the arrow indicator.
Turn the cover until it stops, against any difficulty owing to the waterproof seal.
The ProStaff Five is identified as a rangefinder.
On the other hand the model 6751 is a scope/sight.
I have found the documentation for it. And it does not mention anything about a battery.
You may have to take it to a shop, unfortunately. It may also have a defective light, as well as / instead of a defective battery.