Microsoft Word in Office 2010 can be activated after trial period using the Licensing Operations option available as part of the Help menu.
To activate the product, click Help menu and navigate to Trial Product section.
Click the drop down arrow available next to the Licensing Operations. This would display the Activate Product and Purchase Product link.
Click Activate Product link, this would display the following Microsoft Office Activation Wizard
Click Upgrade button to launch Office Anytime Upgrade window.
If you have the license key for Office 2010 then choose I already purchased Office 2010 option to launch the Enter Your Product Key for Office 2010 window.
After entering the product hey, click Continue button to validate and confirm the Product Key.