IDA: How did you try to change it?

Question

I am VERY new to reverse engineering but I find it fascinating. I want to change the following code:

cmp     [ebp+arg_0], 2  
jnz     short loc_41143B  

I would like to change the 2 to any other number, but when I try to change it to a 3 IDA says invalid operand size(s) when I use the edit assembly feature in IDA.

Why doesn't it accept say "cmp [ebp+arg_0], 3" ?

The source code says:

int main(int argc, char *argv[]) {
volatile int a = 0x0a;
volatile int b = 0x27;
volatile int c = 0x3b;
volatile int d = 0x63;
if (argc == 2) {
    a ^= 0x42;
    b ^= 0x42;
    c ^= 0x42;
    d ^= 0x42;
    printf("%c%c%c%c\n", a, b, c, d);
}
return 0;

}

Answer 1

So you don't use change assembly, you use change bytes in the IDA edit menu.

So if you want to go from 2 to 3 you change the bytes: Original:
83 7D 08 02 75 4B 8B 45 F8 83 F0 42 89 45 F8 8B

Changed:
83 7D 08 03 75 4B 8B 45 F8 83 F0 42 89 45 F8 8B

Also, if you want to change the argc == 2 to say argc > 2 then this changes:
jnz short loc_41143B to jle short loc_41143B