Completely normal (if one follows Bernstein) is 4.3 - 4.6, definitely 5.0 or less.
5.6 / 5.7 and above will get you diagnosed as prediabetic / impaired glucose tolerance.
6.5 and above will get you diagnosed as full-blown diabetes.
Even if you are diabetic, the healthiest range would be as close to non-diabetic levels as possible. Certainly in the 5s. Hopefully, the low 5s.
Note; this is different from the official recommendations, that say that 6.5 - 7.0 is ok for persons with diabetes.
However, levels of 6.5 - 7.0 will result in diabetic complications over time.
How to normalize blood sugars:
- Lowcarb diet (Paleo / keto/ modified atkins,)
- Weight management
- exercise
- medications, if necessary and levels cannot be normalized with the first 3 approaches.