Give A Right Click On The Internet Icon And Go To Properties

Next Window Will Be Like This

Click On CONTENT

Click On The CERTIFICATES And The Window Will BE Like This

Select The Certificate Which You Want To EXPORT And Click On The EXPORT Button

Click Next To Proceed Further

Select No Do Not Export The Private Key And Click Next

Select The First Option And Click Next

Click On BROWSE

Next Window Will Be

Select The Desktop

Mention The File Name At The Space Provided ( in whose name you have taken the certificate) And Click On The Save Button

Click Next

Click On Finish

Click On Ok

Click Close

Click OK

Now On Your Desktop There Is An Icon Of Green Colour Which Is Your .cer Format Or The Public Key.