From LSAC:
If you would like to replace the letters of recommendation on file with new ones, send a signed, written request indicating exactly which letter(s) need to be de-activated.
Write to LSAC-Document Processing, 661 Penn Street, Box 8508, Newtown, PA
18940-8508, or Fax 215-504-1444.
Include your name, social security number, and signature.