My purchases are shown as cost of sales in profit and loss account. i wish that it should show as purchases. how can i change it ?

It's possible that the account assigned in the Expense account field when your product was set up is Cost of Sales, pkadhiwala.

Let review the details and change the account to a regular expense by following these steps: 

1. Go to the Gear icon and select Products and Services.
2. Look for the item or product that you purchased and click the Edit button.
3. Scroll-down a little bit to the Expense account section.
4. Click the drop-down list and change it to the correct purchase expense account. 
5. Click Save and close. 

If the items that you purchased are not set u in your item list, make sure that the account used in the Purchase Bill is a purchase account. Just open the transaction and proceed to the Category details section. Then, change the account used before clicking Save and close. 

You'll also want to check out this article for additional reference when preparing your year-end tasks: QuickBooks Online Year-end Guide And Checklist.

The Community is always here if you need more help with your reports.