Equivalent gore, without shitty variable names or nested ternaries.

int changeVolume(int currentVol, bool up) {

int adjustment;

if (currentVol % 2 == 0) {
    //is even

    int adjustOneModResult;

    if (up) {
        //we increment one only if we're on 4, 14, 24, etc ...
        adjustOneModResult = 4;

    } else {
        //we decrement one only if we're on 6, 16, 26, etc ...
        adjustOneModResult = 6;
    }

    if (currentVol % 10 != adjustOneModResult) {
        // even, not on 4, 14, 24 if going upwards, and not on 6, 16, 26 if going downards
        if (up) {
            // even, going upwards, and not on 4, 14, 24, etc ...
            adjustment = 2;

        } else {
            // even, going downards, and not on 6, 16, 26, etc ...
            adjustment = -2;

        }

    } else {

        // even, on 4, 14, 24 if going upwards, or  on 6, 16, 26 if going downards
        if (up) {
            // even, going upwards, on 4, 14, 24, etc ...
            adjustment = 1;

        } else {
            // even, going downards, and on 6, 16, 26, etc ...
            adjustment = -1;

        }

    }

} else {
    //is on odd multiple of 5
    if (up) {
        // is on odd multiple of 5 going upwards            
        adjustment = 1;

    } else {
        // is on odd multiple of 5 going downwards
        adjustment = -1;

    }

}

return currentVol + adjustment;

}