Create a select query that pulls the records to be updated.  Run it and
review ( better yet is to backup the database first  ).
Say the your field is "GardenTops", so in the design view of the query,
click on the icon that looks like two datasheets with a down arrowhead to the
right. Select Update Query.
In the Update To row put this ---
      [GardenTops] * -1
If you have some that are negative now and do not want to change to a
positive number then add criteria of >0  (greater than zero),

It will multiply all seleced records by -1 and replace what is there.