Currently I already have acroread version 4.0 on my laptop
but I am going to upgrade to 5.0

goto www.adobe.com and download the linux version

you'll get a file
linux-508.tar.gz

unpack it by
>$ tar -xzf linux-508.tar.gz

view the README first (to make sure you understand what you're doing)

become root
>$ su

then run the INSTALL script
>$ ./INSTALL

follow the instructions and you're done...

you may have to remove Acrobat4
>$ apt-get remove acroread

then you may have to place acroread in your bin again (for Acrobat 5)
>$ ln -s /usr/lib/Acrobat5 /usr/bin/acroread

you're done, get out of root
>$ exit

and run acrobat 5
>$ acroread