Producing a pyramid of capital letters
It's been a while since I coded in C but here are couple observations.


Input validation. You should check if the scanned character is really an upper case letter or exit with an error otherwise.
Choice of functions. Since you're scanning and printing out a character at a time, you could have used putchar instead of printf, and getchar instead of scanf.
Variable naming. let is not the best variable name since it is often used as a keyword in other languages. You also could have used the same variable in place of let and back.
Code formatting. There are a lot of different opinions about how to format source code, but most agree that it is better to put parenthesis around loop and condition bodies.


Here is how I would have formatted your code.

#include &lt;stdio.h&gt;

int main(void) {
    char ch;
    char row;
    char spaces;
    char letter;

    printf("Please, insert an uppercase letter: ");
    scanf("%c", &amp;ch);

    for (row = 'A'; row &lt;= ch; row++) {
        for (spaces = ch; spaces &gt; row; spaces--) {
            printf(" ");
        }
        for (letter = 'A'; letter &lt; row; letter++) {
            printf("%c", letter);    
        }
        for (; letter &gt;= 'A'; letter--) {
            printf("%c", letter);
        }
        printf("\n");
    }
    return 0;
}