set iphone calendar to default to weekly view

I have an iPhone 5c. The iOS is up to date. The calendar was set to the weekly view, which is my preference. From time to time the view changes to a monthly one: I'm not sure what occurs to effect this change. Sometimes I can switch it back to the weekly view, but I'm not sure how I do this. I deleted the calendar app and reinstalled it, and this time it switched back to weekly. Could anyone advise how I can change between weekly and monthly views within the calendar itself?. Many thanks in advance for your help.