Help with modulo. 25%10 always rounds off to 6.

works for me, too:
output is exactly what you intended

int ones, tens;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);

}

void loop() {
  // put your main code here, to run repeatedly: 
  for (int i=0; i<100; i++){
    processNumber(i);
    Serial.print(i);
    Serial.print("\t");
    Serial.print(tens);
    Serial.print("\t");
    Serial.println(ones);
  }
while (true){};
}

void processNumber(int number)
{
  int x;
  // unsigned int ones;
  // int tens;

  ones=(number)%10;
  x=(number)-ones;
  tens=x/10;

  // updateDisplay(ones,tens);
}

output:

0	0	0
1	0	1
2	0	2
3	0	3
4	0	4
5	0	5
6	0	6
7	0	7
8	0	8
9	0	9
10	1	0
11	1	1
12	1	2
....
88	8	8
89	8	9
90	9	0
91	9	1
92	9	2
93	9	3
94	9	4
95	9	5
96	9	6
97	9	7
98	9	8
99	9	9