If your mask is too big to fit securely on your face, take the following steps.
First, make sure you’ve washed your mask. This is important for best safety practices and will also help make the mask snug on your face.
Then, the simplest thing to do is tie a small knot on each ear loop to shorten them. This results in some of the excess elastic being tied behind each ear.
Again, some customers have modified their masks in a way that resembles swimming goggles by tying the end of the string/elastic from one ear loop to the other and tightening it around the back of the head.
Get a household object (shoestring or elastic band) and feed it through both ear loops. Then secure around your head to the appropriate tightness level. This should keep the mask taut on your face, but be comfortable to wear.
If you would like to purchase a product specifically for this use, rather than using something at home, search 'ear saver' extenders online.