Look up Word Help for field switches, e.g. so instead of

{ MERGEFIELD mydate }

you use

{ MERGEFIELD mydate \@"DD MMMM YYYY" }

Alternatively, revert to the old method (DDE) of link to Excel - check Word
Tools|Options|General|"Confirm conversions at open", go through the process
of connecting to your sheet again, and choose the DDE option when prompted.
This method is "deprecated", requires Excel on your system, and only works
with the sheet that excel opens by default - typically the first sheet in
the workbook.

Peter Jamieson