Open the Settings app
Tap Mail
Tap Accounts
Select the email account you want to change
Tap Account (or the email address/account name)
Tap Password
Enter the new password
Tap Done
If prompted, tap Save
If the account doesn’t show a Password option:
Open the Settings app
Tap Passwords
Use Face ID/Touch ID or enter your passcode
Find the email account entry
Update the password to the new one
Tap Done
If you use a provider account (e.g., Google, Microsoft, Yahoo):
Go to the provider’s website or app to change the password
Return to the iPhone Mail account settings
Update the password as above
If you still can’t sign in:
Remove the account: Settings > Mail > Accounts > select account > Delete Account
Add it again: Settings > Mail > Accounts > Add Account
Enter the new password