Hopefully you already know how to change a fraction to a decimal.
For instance ²⁄³ =2 ÷ 3= 0.6666… This is a recurring decimal. But changing a recurring decimal back to a fraction is a little more complicated.
UK maths teacher have produced a great video on this.
If you prefer a written explanation Study maths have produced a good introduction of how to convert a recurring decimal to a fraction.