Open the Contacts app or Phone app
Find the contact you want to remove
Tap the contact
Tap Edit
Scroll down
Tap Delete Contact
Tap Delete Contact again to confirm
To delete multiple contacts, use iCloud.com on a browser
Sign in with your Apple ID
Open Contacts
Select the contacts
Press Delete on your keyboard
Confirm the deletion
To stop syncing contacts from an account, go to Settings
Tap Apps
Tap Contacts
Tap Contacts Accounts
Select the account
Turn off Contacts
Choose Delete from My iPhone if prompted