In order to copy the files from the desktop to the mobile device, you’ll have to have Windows Mobile Device Center installed on your PC.

Windows Mobile device Center:

Windows Vista, 7, 8 & 8.1 64-bit: https://www.microsoft.com/en-us/download/details.aspx?id=3182
Windows Vista, 7, 8 & 8.1 32-bit: https://www.microsoft.com/en-us/download/details.aspx?id=14
Windows XP (Active Sync): https://www.microsoft.com/en-us/download/details.aspx?id=15

Once installed, connect the device's cradle to you PC and place the device in the cradle. Windows Mobile Device Center should start and bring you to a connected screen. If it does not, try rebooting you PC. After the reboot is complete, start Windows Mobile Device Center (Start - Programs) and cradle your device.

Only one device can be connected to a PC at a time.

Open Windows Explorer and browse to the .cab file that you want to install on the device. Right click on the right click file and select “Copy”.
Select “My Pocket PC” and in the left panel, right click and choose “Paste”. This will copy the file from the desktop to the mobile device.

Depending on the device and OS, the device may be named differently. However, it will show up as a drive or device in "My Computer"
After the file has completed copying, open the devices File Explorer from the Programs menu on the Mobile Device, and go to the mobile devices root directory, and click on the .cab file that you copied to the device. This will start the installation process.