Here is the link. You must download both factory-to-ddwrt.bin & tl-wr740nv4-webflash.bin.
1. Enter 192.168.1.1 from IE to login to the router. Select System Tools then click on Firmware Upgrade.
2. Choose factory-to-ddwrt.bin which you download to your computer previously.
3. Click Upgrade button to start firmware upgrading.
4. The router will restart automatically after completed upgrade.
Screen shot for upgrade firmware: http://www.divshare.com/download/18183284-fed
5. Login to router again. Now DD-WRT prompt you for changing user name & password.
6. Enter the new router user name & password then click on Change Password button.
7. Enter new user name & password to login to the router.
8. Click on Administration from DD-WRT menu then select Firmware Upgrade.
9. Choose tl-wr740nv4-webflash.bin & click on Upgrade button
The router will upgrade to DD-WRT dated 03-19-12-r18777 which is much more powerful than before.