The six-minute video above reviews the meaning of fractions and then explains fraction addition. (If you want to skip to the review, start the video at 2:30.)
The video above explains how to subtract fractions.
When you change a denominator (the bottom part) of a fraction, you must also change the numerator (the top part) to keep the fraction worth the same amount.
Practice and study
Use these search terms to find your own resources:
adding fractions unlike denominators
subtracting fractions unlike denominators