Open the Contacts app or the Phone app
Tap the contact you want to delete
Tap Edit
Scroll down and tap Delete Contact
Tap Delete Contact again to confirm
Repeat for each contact you want to remove
To delete multiple contacts, use iCloud on a computer
Sign in to iCloud.com
Open Contacts
Select the contacts you want to delete
Press Delete on your keyboard
Confirm the deletion
To remove all contacts from iPhone, turn off Contacts sync for your accounts
Go to Settings
Tap your name
Tap iCloud
Turn off Contacts
Choose Delete from My iPhone if prompted
Repeat for other accounts in Settings > Contacts > Accounts