Like mentioned in a comment skip everything between Z for example this way:
#include "stdio.h"
int main(){
    char ch = 'A';
    while(ch <= 'z'){
        if(ch <= 'Z' || ch >= 'a'){
            printf("%c", ch);
        } 
        ch++;
    } 
    printf("\n");
    return 0;
}
or alternatively use a for loop since you know the beginning and end of the range of values:
#include "stdio.h"
int main(){
    char ch;
    for(ch = 'A'; ch <= 'z'; ch++){
        // loop through range of ASCII values from 'A' to 'z', upper case
        // and lower case letters, and print out only alphabetic characters.
        if(ch <= 'Z' || ch >= 'a'){
            // character is in range of 'A' thru 'Z' or 'a' thru 'z'.
            printf("%c", ch);
        } 
    } 
    printf("\n");
    return 0;
}
or alternatively use the isalpha() function to detect if a character is alphabetic or not.
#include "stdio.h"
int main() {
    char ch;
    // loop through range of ASCII values from 'A' to 'z', upper case
    // and lower case letters, and print out only alphabetic characters.
    for (ch = 'A'; ch <= 'z'; ch++)  isalpha (ch) && printf ("%c", ch);
    printf("\n");
    return 0;
}