I Reply myself, since it could be usefull for some newbie.
I tryed to collect infoes I got from man instalboot and from google.

How to update boot block on NetBSD/ALPHA expecially from 1.5 to 1.6
in my case:

cd /usr/src/sys/arch/alpha/stand
make
rm /usr/mdec/* /usr/share/man/man8/alpha/*
make install


then lets suppose your boot disk is sd0.
Take the whole disk (The c slice) as the argument for installboot program.
It would be more correct to use sd0a instead of sd0c but NetBSD refuses
for security reasons to write into a raw device corresponding to an
already mounted partition. So we will use sd0c which means the entire
disk. This is the correct command to install a primary bootstrap on sd0.
We need to use the raw device rsd0c:

/usr/sbin/installboot -v /dev/rsd0c /usr/mdec/bootxx_ffs

Then it's also better to install the secondary bootstrap, this is done by
copying boot into the /

cd /usr/mdec
cp boot /

and all is done


Rick