Hi Walid.

It's in Switzerland. Typically it receives referrals from the various local TAG-Heuer representatives rather than directly:

http://www.tagheuer.com/the-brand/contacts-services/tag-heuer-worldwide/index.lbl?lang=en

Get in contact with whichever is nearest to you and they should be able to give you details of the process.

Alternatively, as Mark suggests, you could try an independent watchmaker - a number of those frequent this forum and some specialise in Heuers. A lot depends on where you are, as to whether it makes more sense to use one of them or TAG-Heuer themselves.