Yahoo is web-based email service, you can use this service through your iPhone, but if you are using it and you have to Change Yahoo Password on iPhone then you can change it by following some steps.
Change yahoo email password
Open Yahooapp on your iPhone,
Go to the side barand tap on it, it is on top left,
You have to scroll down and tap on “account info”
Then tap on “Account Security” and tap “Change security”
Then type the new passwordof 8-32 characters and then re-type the same passwordin the dialog box which is given there.