Take a look at the mySQL manual for that stuff, there's a somewhat hidden entry for LOAD DATA IN FILE.
All you really need is exporting your XLS file to a comma-delimited (or "tab-delimited") file, e.g. "products.csv".
Then upload that file, either through an HTML-form in an admin area, or trough phpmyAdmin.
Here's a basic example:
$sql = " DELETE FROM $table " ;
@$result = mysql_query($sql) or die (' Couldn\'t delete existing table contents.');
$sql2 = " LOAD DATA LOCAL INFILE '$myFile' INTO TABLE $table " ;
@$result2 = mysql_query($sql2) or die (' Could not insert anything. Sorry.');
This throws out the old data completely, and inserts data from your textfile.
You might want to do a backup table first.
Also make sure of course, that your tables really match (structure, columns) etc.
It's so superfast and easy, I was yodeling when I discovered that nifty feature