You can delete a non-integrated account when no opportunities are associated to the account. Integrated accounts can only be modified in the source CRM system (Salesforce, Hubspot, etc).
To delete an account,
Only account owners have the ability to delete an account. Accounts with attached opportunities cannot be deleted.
Click EXECUTE on the menu bar and select Accounts.
Click the kebab menu () for the account you want to delete.
Select Delete from the options available.
The Accounts table will reflect the recent changes.