#include <SD.h>

#define SD_CS     4

void setup() {
   
  File testFile;
  char strA[] = {"aaaaaaaaaaaaaaaaaaa"};
  char  strB[] = {"bbbbb"};
  char  buff[25];

  Serial.begin(9600);
  if (SD.begin(SD_CS)) {
    testFile = SD.open("/testFile", FILE_WRITE);
    if (testFile) {
      testFile.write(strA,20);
      testFile.close();

      testFile = SD.open("/testFile", FILE_READ);
      testFile.read(buff,20);
      buff[20] = '\0';
      Serial.println(buff);

      testFile.close();
      testFile = SD.open("/testFile", O_WRITE);  // Open for write and NOT append..
      if (testFile.seek(5)) {                       // <<== seek() to 5..     
        testFile.write(strB,5);
        testFile.close();

        testFile = SD.open("/testFile", FILE_READ);
        testFile.read(buff,20);
        buff[20] = '\0';
        Serial.println(buff);
      } else {
        Serial.println("seek() failed.");
      }
    }  
  } 
}

void loop() {  }