You are most likely at a pixfirewall# prompt (or whatever your pix hostname is) type:

! Move into configuration mode
config t
!
! Add new DNS reference.
vpngroup vpnusers dns-server 10.10.10.2
!
! Remove old DNS reference (if required)
no vpngroup vpnusers dns-server 10.10.10.1
!
! Test
!
! Save cu...

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