u need only 5mb of space on ur root filesystem to upgrade to aix 5 L .
change the bootlist by using command
#bootlist -m service cd0 rmt0 hdisk0
now restart and press F5 or 5 whatever ur machine supports.u will get three options 1.)complete overwrite install,2.)migration,3.)preservation..its better if u choose migration install because in preservation install ur /usr ,/,/tmp,/home filesystems are overwritten.but in migration install only /tmp filesystem is overwritten.all the device drivers and softwares are in /usr filesystem.
be careful take mksysb backup first..