Switch#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#enable password ?
  0      Specifies an UNENCRYPTED password will follow
  7      Specifies a HIDDEN password will follow
  LINE   The UNENCRYPTED (cleartext) 'enable' password
  level  Set exec level password

Switch(config)#enable password admin
Switch(config)#exit
Switch#write memory