Teach your grandma how her phone works without sighing. She taught you how to hold a spoon.

Impatience with someone learning something new is mostly forgetting that you were once the one being taught. Every obvious thing you know about a phone was explained to you at some point, usually by somebody who had to say it more than once. The sigh is the part that gets remembered, long after the settings menu is forgotten.

Sit down and go slowly next time. Write the steps on paper so she can use them without you there. Let her do the tapping instead of taking the phone out of her hands. Being taught badly is what makes people give up.