Your printf/scanf formats are mixed up, and you're using floats as array subscripts.
Also, the indentation could be a lot better.
Code:
#include<stdio.h>
//!!#include<conio.h>
float average(float[], float);
float highest(float[], float);
float lowest(float[], float);
int main(void)                  //!! fixed
{
  float temper[15];
  float x, n, y, z;
  int day;
//!!clrscr();
  printf("enter the no.days to read temperature for=\n");
  scanf("%d", &n);
  for (day = 0; day < n; day++) {
    printf("temperature for day%d=", day + 1);
    scanf("%f", &temper[day]);
  }
  x = average(temper, n);
  printf("average temperature is=%.2f\n", x);
  y = highest(temper, n);
  printf("highest temperature is=%.2f\n", y);
  z = lowest(temper, n);
  printf("lowest temperature is=%.2f\n", z);
//!!getche();
  return 0;                     //!! added
}

float average(float temper[], float n)
{
  int sum = 0.0, x;
  float average;
  for (x = 0; x < n; n++)
    sum += temper[x];
  average = sum / n;
  return (average);
}

float highest(float temper[], float n)
{
  float highest;
  int y;
  highest = temper[0];
  for (y = 0; y < n; y++)
    if (highest < temper[y])
      highest = temper[y];
  return (highest);
}

float lowest(float temper[], float n)
{
  float y, lowest = temper[0];
  for (y = 0; y < n; y++)
    if (lowest > temper[y])
      lowest = temper[y];
  return (lowest);
}
Here are the errors that need to be fixed
Code:
$ gcc -Wall bar.c
bar.c: In function ‘main’:
bar.c:13: warning: format ‘%d’ expects type ‘int *’, but argument 2 has type ‘float *’
bar.c: In function ‘lowest’:
bar.c:53: error: array subscript is not an integer
bar.c:54: error: array subscript is not an integer