good day!
i just got my N810. how can i install application to our N810 if i downloaded the file on my pc?

tnx

Sometimes you can simply move that file to your memory card on the NIT and use Application Manager to install it from a file. But it is often easier to to navigate to the site with the Application in question and install it directly to the Tablet.
You can find many tips at the internet tablet school

Make sure that there is enough charge in your N810. Connect the N810 to your PC using the USB cable that came with the N810. Once the PC recognizes the N810 as an external Drive, simple yank the files that you downloaded, into the N810. Place these files in the Folder you wish to save them into. Make sure that the flies you download have the necessary required extension so that it is compatible with the Nokia 810 OS.

I assume that the file is a Nokia Internet Tablet (NIT) application file that is for your N810. If so, then create a folder on your memory card called "install" or whatever and transfer your file to that folder (either through the USB cable or using a SD card reader). Then run the application manager, click on top left of screen "Application Manager", then click "Application", then click "Install from file". Finally browse to the install folder for the file.