How do I replace a caracter in a string?

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main()
{
	int x;

	printf("Pick the program that should be executed:\n");
	printf(" 1. Split text\n 2. Upper case to lower case\n 3. Lower case to upper case\n 4. Remove a character\n 5. Add a character\n 6. Replace a character\n 7. Statistics\n 8. Exit\n Enter an option:\n");
	scanf("%d", &x);

	if (x == 1)    ///split the text 
	{
		char str[100];
		int i;

		printf("Write the text that should be used:\n");
		getchar();
		fgets(str, 100, stdin);

		printf("Input was: %s\n", str);

		for (i = 0; str[i] != '\0'; i++)
		{
			printf("%c", str[i]);
			printf("\n");
		}
	}

	if (x == 2)   //upper case to lower case
	{
		char str[100];
		int i;

		printf("Write the text that should turn upper cases to lower cases: \n");
		getchar();
		fgets(str, 100, stdin);

		printf("Input was: %s\n", str);

		for (i = 0; str[i] != '\0'; i++)
			if (str[i] >= 'A' && str[i] <= 'Z')
				str[i] = str[i] + 32;     //A - Z = 65 - 90, a - z = 97 - 122

		printf("string converted to lower case: %s", str);

		getchar();

	}

	if (x == 3)  //lower case to upper case
	{
		char str[100];
		int i;

		printf("Write the text that should turn lower cases to upper cases\n");
		getchar();
		fgets(str, 100, stdin);

		printf("Input was: %s\n", str);

		for (i = 0; str[i] != '\0'; i++)
			if (str[i] >= 'a' && str[i] <= 'z')
				str[i] = str[i] - 32;

		printf("string converted to upper case: %s\n", str);

		getchar();
	}

	if (x == 4)
	{
		char str[100];
		size_t i, j, len;
		int r;

		printf("Enter a sentence: \n");
		fgets(str, 100, stdin);
		if (fgets(str, 100, stdin) == NULL) 
		{
			fprintf(stderr, ("fgets failed"));
			return -1;
		}

		str[strcspn(str, "\n")] = 0;
		printf("This is the sentence: %s\n", str);

		printf("Enter character to remove: \n");
		r = getchar();

		/*If getchar returns EOF, no need to go through character removal logic*/
		if (r != EOF) {
			len = strlen(str);

			i = 0;
			while (str[i] != '\0') {
				if (str[i] == (char)r) {
					for (j = i; j < len; j++) {
						str[j] = str[j + 1];
					}
					len--;
				}
				else
					i++;
			}
		}

		printf("Sentence after removal: %s\n", str);
	}
	if (x == 5)
	{
		char str[100] = "";
		size_t len;
		int r;

		printf("Enter a sentence: ");       
		fgets(str, 100, stdin);
		if (fgets(str, 100, stdin) == NULL)
		{
			fputs("fgets failed\n", stderr);  
			return 1;  
		}

		str[(len = strcspn(str, "\n"))] = 0;    //save len 
		printf("This is the sentence: '%s' (len: %zu)\n", str, len);

		if (len < 100 - 1)
		{
			printf("\nEnter character to add: ");
			if ((r = getchar()) != EOF) 
			{
				str[len++] = r;
				str[len] = 0;
				printf("This is the sentence: '%s' (len: %zu)\n", str, len);
			}
			else
				fputs("(user canceled input.)\n", stderr);
		}
		else {
			fputs("error: insufficient space to add char.\n", stderr);
			return 1;
		}
	}
	system("pause");
	return main();

	if (x == 6)
	{
		char str[100];
		size_t len;
		int r, i, j;
		struct newlen
		{
			char str[50];
		};
		
		printf("Write the sentece that should be used\n");
		fgets(str, 100, stdin);
		if (fgets(str, 100, stdin) == NULL)
		{
			fputs("fgets failed\n", stderr);
			return 1;
		}

		str[(len = strcspn(str, "\n"))] = 0;
		printf("Choose the letter that should be replaced:\n");
		r = getchar();

		if (r != EOF) {
			len = strlen(str);

			i = 0;
			while (str[i] != '\0')
			{
				if (str[i] == (char)r)
				{
					for (j = i; j < len; j++)
					{
						str[j] = str[j + 1];
					}
					newlen = newlen.replace(str[j + 1], r);
				}
				else
					i++;
			}
		}

		printf("Sentence after replacement: %s\n", str);

	}
}