How to position the input text cursor in C?

If you are under some Unix terminal (xterm, gnome-terminal ...), you can use console codes:

#include <stdio.h>

#define clear() printf("\033[H\033[J")
#define gotoxy(x,y) printf("\033[%d;%dH", (y), (x))

int main(void)
{
    int number;

    clear();
    printf(
        "Enter your number in the box below\n"
        "+-----------------+\n"
        "|                 |\n"
        "+-----------------+\n"
    );
    gotoxy(2, 3);
    scanf("%d", &number);
    return 0;
}

Or using Box-drawing characters:

printf(
    "Enter your number in the box below\n"
    "╔═════════════════╗\n"
    "║                 ║\n"
    "╚═════════════════╝\n"
);

More info:

man console_codes

In the linux terminal you may use terminal commands to move your cursor, such as

printf("\033[8;5Hhello"); // Move to (8, 5) and output hello

other similar commands:

printf("\033[XA"); // Move up X lines;
printf("\033[XB"); // Move down X lines;
printf("\033[XC"); // Move right X column;
printf("\033[XD"); // Move left X column;
printf("\033[2J"); // Clear screen

Keep in mind that this is not a standardised solution, and therefore your code will not be platform independent.

The C language itself doesn't have any notion of a screen with a cursor. You'll have to use some kind of library that provides this support. curses is the most well-known and widely available library for terminal control.