code: public StrBuilder replaceFirst(final char search, final char replace) {
          if (search != replace) {
              for (int i = 0; i < size; i++) {
                  if (buffer[i] == search) {
                      buffer[i] = replace;
                      break;
                  }
              }
          }
          return this;
      }
docs: Replaces the first instance of the search character with the
  replace character in the builder.
  
  @param search  the search character
  @param replace  the replace character
  @return this, to enable chaining