I have an upper that came without the dust cover option . I have seen that these cover assemblies can be purchased . Now my question is , how can I install this option ?

[url]www.ar15.com/content/books/TM9-1005-319-23.pdf[/url]

Right click the link and select 'save target as', the file will download as a .pdf file for viewing on your computer.

To install the ejection port cover:

1. Snap the 'C' spring on the ejection port cover pin.

2. Run cover pin into the receiver from barrel end and into right side of ejector port cover.

3. Place the spring so the tail on the barrel end of the cover is on the inside of the cover.

4. Twist spring an extra turn to tighten, hold spring and slide cover pin into the hole at rear of receiver...

edited for .pdf link...

Not sure I understand what you're asking when you refer to the cover as an "option". It's a standard part on most ARs, and the upper has two little tabs just below and to either side of the ejection port, with small holes, to accomodate the retaining pin for the cover. If your upper has those tabs, it's simply a question of procuring the cover, it's spring, and the long pin, and installing them.

There are, however, some match and custom type uppers on the market that were not intended to take a cover.  They do not have the retaining tabs, and you cannot mount a covere on them.

If you have the first (usual) type upper, let us know, and we'll give you info on mounting. BTW, also let us know if the upper has a barrel mounted or is stripped.

My upper has no tabs to mount traditionally . This is fully assembled rifle .

Well, I guess you answered your own question, unless you want to replace the upper with a traditional type.