uploading to a mega

I have several mega's and have recently bought another two. I cannot get the program to upload to the mega micro processors that I have just bought. My computer sees them as 'com 6' devices. When I connect my older Megas to the USB port they work fine: they are seen by my computer as com 5 devices. When I go into the arduino tools the 'port' option is in grey rather than black so I can't change the com device that the upload software requires.How can I free this com number option up?