First you need to do is get emmc file, you will find it in: http://tablets-dev.nokia.com/nokia_N900.php
and download RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin

After downloading emmc then download:
RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin
now open maemo flasher-3.5.exe and type flasher-3.5 -F RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin -f -R
after it flashing is done don't switch on your mobile.
then type flasher-3.5.exe -F RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin -f -R
It will give you the latest pr 1.3 in your n900, hope this will help.