Open the Gmail app on your iPhone
Tap your profile picture in the top right
Tap Manage your Google Account
Swipe to the Security tab
Tap Password
Sign in if prompted
Enter your new password
Confirm the new password
Tap Change Password
If needed, update the password in iPhone Settings
Open Settings on your iPhone
Tap Mail
Tap Accounts
Tap Gmail
Tap Account
Enter the new password
Save the changes