Are you in the UK? If so, see: Changing or adding a parent's details to a birth record : Directgov - Government, citizens and rights (http://www.direct.gov.uk/en/Governmentcitizensandrights/Registeringlifeevents/Birthandadoptionrecords/Registeringorchangingabirthrecord/DG_175620)

Can you explain the situation a little better so we can help you ?

It depends completely on your situation and where you are located.

Generally there are two ways:
1.Through court procedure
2.A father may sign Paternity acknowledgment.