You can't actually reset your wifi password for your router from your phone that i'm aware of.
But for example if you've changed your wifi password, and you want to update the changes on your phone
1. open settings
2. tap wifi
3. tap the i in the circle to the right of your network's name and choose "forget network"
4. turn wifi off and back on
5. tap network name and type in new password
Hope this helps.