how do you change low power mode

As the name suggests "Low Power Mode" will work only when the power is low. Isn't it obvious?

• When Low Power Mode is on, the battery in the status bar will be yellow. You'll see  and the battery percentage. After you charge your iPhone to 80% or higher, Low Power Mode automatically turns off.

It's by design, as correctly explained by SravanKrA, but if you don't like it, you can submit your request/suggestion to Apple directly, using this link:--> Apple – Product Feedback - iPhone

Regards

Giulio