By year 6 children will need a clear understanding of equivalent fractions. They should understand that 1/2 is equivalent to 2/4, 6/8 etc. This will help them enormously when they come to adding and subtracting fractions; in fact they won’t be able to do them without this understanding.
A fraction such as 5/20 can be reduced to an equivalent fraction 1/4 by dividing the numerator and denominator by the same number. This is usually known as cancelling.
A fraction such as 4/10 can be changed to an equivalent fraction by multiplying both the numerator and the denominator by the same number. There is no mathematical term for this, but it is often known as lecnacing (cancel backwards).