Converting ADC to voltage
It depends on the binary format the A/D produces.  Let's say it does the basic thing and produces a unsigned 14 bit value.  That means -10 V results in 0, and +10 V in 214-1 = 16383.

Even if the A/D output is mapped differently into the 16 bit word, this is what the A/D is doing internally.  You should be able to recover the raw 14 bit value by shifting and masking, and possibly flipping a bit.

You can also put in a few known voltages and see what comes out.  The binary format should be obvious after looking at a few examples.

To reverse this process, linearly scale 0 to 16383 to map to -10 V to +10 V.

This should, of course, both be obvious and clearly spelled out in the datasheet.