According to the manual on ipdb, showing all 5's is an indication of a factory reset due to battery failure.
I've no actual hands-on experience with the keyboard set-up, but (again according to the manual), rather than manually stepping through the book-keeping functions by pressing the test button, the button's only used to get into the test functions. After doing that, the required function or 'register' number is entered on the key pad. The Highest Score is register 15.
So, as I read it,
- Open door and press the test button (hopefully the keyboard itself will pass a test made at this point)
- Specify register 15 by entering 1, 5 and enter on the key pad
- Match display shows the register number, and player 1 shows the current value
- Enter a new value using the key pad; the default is stated to be 1,999,990. Whatever you choose, it appears in player 2 display
- Press enter to actually alter the stored value. Both score displays show the same (new) value
- Step out of adjustment by pressing Game