Are your values in that column Text or are they numbers formatted to show the
dash?

If they're text
select the column
data|text to columns
delimited by -
skip (do not import that second column)
Put it right back in the same location.
format|cells|number tab
custom
00000
(to keep leading 0's showing)

Or use a helper column with formulas like:

=left(a1,5)

======
If they're really numbers formatted to look that way:

I'd use a helper column
=INT(A1/10000)

And format that nicely.

Manu wrote:

Column contains 10 characters (e.g. 02453-1234) making up the zip+4. I need
to remove the last 5 characters, leaving only the basic 5 digit zip-code.
The column contains about 10,000 entries. Please suggest methods by which
this can be done. Thanks!

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Dave Peterson