Q about using tar to do backups

I'm using tar to create a complete system backup.
That works just fine but there doesnt seem to be a
true "update" option to tar (or have i misunderstood this opt?).
I really dont want to recreate the tar file each time, all I want
to do is to have it updated if some of the files in the various
dirs change or new files are added.
Suppose I have an existing 500 meg tar file of directory /home/User1,
and the next backup comes due and User1 has 5 files that changed. I
dont want to delete the old 500 meg tar file but i would like to replace
(not append a second copy as "-update" seems to suggest) the changed files
in that tar.
I think you see what I'm getting at.
Is this do-able with a script? Would it be faster (hopefully) than
simply erasing the old tar and creating a new one?
Any ideas, bash snippets, suggestions, etc?
Thanks
Eric

--
57 channels and nothin's on