Delete preloaded sample data in new Salesforce organizations

Here's how to Delete Trial Data to ensure that it is not confused with real data:
 

Option 1

Delete all data if your org is still in the 'Trial' status:
 

1. Navigate to Setup.
2. Enter Delete into the 'Quick Find' box.
3. Select Delete All Data.

Note: The 'Delete All Data' tool will delete ALL data within your organization. Once deleted, this data cannot be recovered. Do not use this functionality unless you wish to delete all data.

Option 2

System administrators of both trial and paid subscriptions can mass delete data. To mass delete only sample data, follow the steps below:
 

1. Locate a sample record (any Account or Contact will do). Find the 'Created By' stamp on the Page Layout. Copy the date and time. All sample data is created at the same time (the time at which the org is created).
2. Navigate to Setup.
3. Enter Delete into the 'Quick Find' box and select Mass Delete Records from the results.
4. Select a type of Record that you wish to delete, such as Accounts. Review the notes which state the records that will be deleted.
5. Under Step 3, enter the following filter: Created Date equals [paste the Created Date from the sample record].
6. Review the conditions of Steps 4 and 5, then click Search. All records which meet your criteria are returned.
7. Scan the list to confirm that you would like to delete all of the records listed.
   Select the checkbox next to all records that you would like to delete, then click Delete.

Repeat this process for other types of records you'd like to delete.

Notes:

• Accounts have related records or child records, such as sample Case. To successfully delete sample Accounts, it is best to delete child records (such as sample Case) first before deleting sample Accounts.
• Account child record such as Opportunities cannot be deleted using Mass Delete Record tool. You can use Data Loader or Dataloader.io to delete this child record and then proceed on deleting the Parent Account records.

See also:
Reset an organization to default 

Suport Video:
How to Delete Preloaded Sample Data in New Salesforce Organizations