Replacement character in C language, simple version

This is compiled by me. The program requires that, when a user input any string containing a, it should be replaced with B. If it is "AA", it should be replaced with "B2 ", the program is very simple and I hope it will be helpful for beginners to learn.

# Include <stdio. h>
# Include <stdlib. h>
Void fun (char a [])
{
Int I;
For (I = 0; A [I]! = '/0'; I ++)
{
If (A [I] = 'A' & A [I + 1] = 'A ')
{
A [I] = 'B ';
A [I + 1] = '2 ';
}
}
}
Main ()
{
Char a [20];
Gets ();
Fun ();
Puts ();
System ("pause ");
}