It depends greatly on what your license method is.

-If you are on subscription it's quite easy just contact your reseller and ask if they can arrange your license transfer, you probably have to fill in a form or something like that.
-With a dongle it's quite easy just copy the aw.dat file from the flexlm folder from your old to your new computer
-With no subscription I would advise you again to contact your reseller /Autodesk and either claim you had a systemcrash and needed to reinstall the computer or tell the truth which will set you back for about $150/200

good luck