I can try to help you if you provide me some more info:
1.How exactly are you trying to update your Reader 8.1.2 ?
2.What is your OS and have you upgraded it recently?
If you have no problem,you can upgrade to free Reader 9.3.4 by following these steps:
-Download 9.3.3 installer http://ardownload.adobe.com/pub/adobe/reader/win/9.x/9.3.3/enu/AdbeRdr933_en_US.exe and run it.
This 9.3.3 installer will automatically uninstall your Reader 8 and will install Reader 9.3.3.
-Download the 9.3.4 update http://ardownload.adobe.com/pub/adobe/reader/win/9.x/9.3.4/misc/AdbeRdrUpd934_all_incr.msp and run it.