If you have download an apk file to your local pc and want to install it to your android device here is what you need to do :

download and install Android SDK according to the SDK install on the android.com site.

if you are running windows , on the sdk install the additional component window sdk driver .
connect your android device using a usb cable.
in the android sdk->platform-tools locate the adb.exe
in command line on the specified path write :

adb install < path to apk >

you should enter your real path to the apk file you want to install on the device.

for more information about installing the sdk and windows driver go to SDK