https://www.diabetes.co.uk/insulin/adjusting-basal-bolus-insulin-doses.html said:
Adjusting your long term, basal insulin dose
Also referred to as background insulin, basal insulin is the insulin which acts over most of the day, typically between 16 and 24 hours depending on which long term insulin you take.
If there is a general trend of higher numbers through the day, it may be advisable to increase your long term insulin instead of adjusting a number of your short term doses.
Conversely, if your numbers are generally going low through the day, you may need to decrease your long term insulin.
If you are unsure about whether your background insulin needs changing, a good way to check is do a fasting test, which involves not eating and not injecting short term insulin for several hours.
Throughout the fast you can test your blood sugar at regular intervals, say once every two hours and observe whether your blood sugars are dropping or increasing