lat9:
Your best bet is to create a new database, copy the v1.5.1 database into that new database, point a Zen Cart v1.5.4 test site at that database and run the zc_install database upgrade procedure. That will make any conversions required in going from v1.5.1 to v1.5.4.
For the OP. This is an equally viable method to use.
The benefit/pitfall of this method is that you won't be messing with the currently installed/working database, but you will need to edit the two configure.php files to 'switch' from the current DB to the newly created one.
The benefit/pitfall of my suggested method is that you won't need to edit the configure files, but you will be overwriting the contents of the currently working database with the content of the original database, IOW, you will be working with 'live' data rather than data in a testing/staging area.
With either method there is a small risk that something could go wrong so always keep backups before doing anything.
Personally I like lat9's suggested method a little better, but it seems to me that more people get themselves into trouble with the configure.php file edits than they do with the export/import.
Pick the method that you feel most comfortable with, and if it doesn't go well, restore from the backup and try the other. :)
As they say, there's more than one way to skin an apple (or something like that) <g>
Cheers
RodG