Depends. Depends how high sugars are, but dietary changes are always helpful (stop regular sodas, juices, candy) and possibly medications.
Find out why! For someone under treatment for diabetes, it is very important to find out why the blood is high. Infections, and other serious illnesses could make the sugars go up. Some meds, especially steroids, can make sugars go up. If the high blood sugar is not a surprise, you can help get it down by avoiding carbs (sugars/starches) in your meals, exercising lightly, and drinking lots of water.
High blood sugar. Most people with this are adults and the vast majority of those are also obese or overweight and may note diabetes in family members. Therefore, losing weight (diet/exercise/proper sleep) is fundamental. If sugars remain high, there are various classes of medications that will correct the problem.