There is no "conversion" from ascii to alphabet. A char is simply a 1-byte integer that is interpreted using the ascii table. Run this little program to see what I mean:
char letter = 'a';
printf("letter in decimal: %d\n", letter);
printf("letter as char: %c\n", letter);
letter = letter + 10;
printf("letter is now %c\n", letter);
You should get:
letter in decimal: 97
letter as char: a
letter is now k
See how the char can be printed as either its character value 'a' or its decimal value 97? And you can do arithmetic on the letter in either form?
If you are trying to convert your a-z into the range of 0-25, remember that
'a' - 'a' equals zero. And
'z' - 'a' equals 25.