How can i round significant figures on a calculator?

You cannot round on a calculator (at least the type I use). You need to learn how to do it yourself, if you are required to round to 3sf look at the 4th digit if it is greater than 5 you round the 3rd digit up and if the 4th digit is less than 5 leave the 3rd digit as it is
eg. 66.7223838 
     = 66.7
eg2. 657892354
     = 658000000

Rounding is done by using the following rules:1. If the digit to the right of place of the place you want to round to is 4 or less, then the digit in the place you want to round to will remain the same and the remaining digits to its right are replaced with zeros.a. We will use 4532 and round to the tens placeb. Find the digit in the tens place. it is a 3.c. Find the digit to its right. That digit is a 2.d. Since the number 2 is less than 4, the digit in the tens place will remain as is and all digits to its right will be changed to a zero.e. 4532 rounded to the nearest ten becomes 4530.
2. If the digit in the place you want to round to is 5 or more, then that digit will increase by one and all digits to its right will be changed to a zero.a. We will use 6876 and round to the tens placeb. Find the digit in the tens place. it is a 7.c. Find the digit to its right. That digit is a 6.d. Since the number 6 is greater than 5, the digit in the tens place will increase by one and all digits to its right will be changed to a zero.e. 6876 rounded to the nearest ten becomes 6880.