1. Find their contact(s). 2. Contact them. Good luck finding contacts, though. LP aren't one to really you know, give their contacts out randomly to the public.
When Chester became sick, he released an e-mail address (putting it here is a risk so I won't), but I doubt that he checks it or uses it.
[email protected] It was used when Chester was sick, but obviously, it's not his email address. If you need to contact them, try contacting LP.com's administrators, they'll be of more help.
you could write to Warner Brothers Records, and directing it to Linkin Park's office, but I doubt it will get screened.