And then you need to copy the tail of the string into the position of the erased character.:)
For example if you may use C standard functions then the code could look the following way
char *p = strchr( s, c );
if ( p ) strcpy( p, p + 1 );
But I think your task is to remove all occurances of a given character. In this case instead of using strcpy you should write your own loop that during the coping to see whether there is one more character that shall be removed.
In C++ there is standard algorithm std::remove. So you can write simply
std::remove( s, s + std::strlen( s ), c );
*std::remove( s, s + std::strlen( s ), c ) = '\0';