(1) MAke backup of your market appliction with titanium backup.
(2) Download 4.0.2.A.0.42 from ( jlmcr87 posted the link )
(3) Flash this firmware with any version of flastool i did this with 0.2.9 & 0.6.8
(4) After flashing the firmware root xperia arc with the methood of ( zergRUSH any version ) i root with the ( zergRUSH version 1 & 3).
(5) After that immediately connect your phone with wireless network and check for update. Its display that new version is availabe 4.0.2.A.0.62. Click to update
After update click to install. IT will take time and your phone start several time dont worry about it. After completing update first boot take time.
(6) To Check root install CWM autoinstaller or Titanium backup. Have fun