Low sugars and not taking insulin or sulfonylurea drugs then it will usually recover itself - but if something (artificial), medications or some other condition is "possibly" causing the low then some quick absorbing carbs (juice is a great example) or glucose tablets should be taken and then re-test in 15 minutes to see if you are in the safe range (above 4.0 mmols or 70+ points)

When blood is high if it's while (fasting - haven't eaten is the past 5 to 6 hours or more) then eating will trigger an insulin response

So while counter-intuitive, adding about 15 carbs to your system if running high while fasting or pre-prandial (before eating) will usually result in bringing your blood sugar lower

If it's high because you just snuck a couple of cookies then drinking water, going for a walk (or more strenuous exercise) - taking a bit more insulin (if you use it) would all help lower it and staying away from food until has returned to your normal pre-meal levels would be in order

From experience the answer is " depends"if low don't take insulin.Eat or use glucose tablets til you get above 4mmol/l( 72).I choose tabs as I am concerned about weight gain.
If high: first thing in morning( FBS) could be result of too long a fast,or dawn phenomenon( hormones make liver release glucose so level rises).Eating will make level come down.If continually high may need to increase basal insulin( long term insulin)If high at other times do not eat. Use insulin,or choose to " let it run".review what caused it if possible .

Thank you Graham and Pandapop