> then : fit f(x) 'data' via a,b,c;
> then i want to store the a,b,c to a file names 'a.log'.
> so i typed : update 'a.log'
That "so" expresses an incorrect conclusion. The 'update' command is
not for creating a new file from scratch. It's for _updating_ an
already existing file, thus the name.
There usually isn't much point in using the 'update' command if your fit
command's "via" argument wasn't a file.
> but i got the wrong prompt :
> gnuplot> update 'a.log'
> Could not rename file a.log to a.log.old
That's not a prompt, it's an error message. It tells you (somewhat
indirectly) that you didn't have a file 'a.log' to be updated.