The replacement battery type is any or equivalent to these:
LR626, AG4, 377 or 177.
The battery is readily available at NurseWatches.com, Dollar Stores, Drug Stores or Walmart.
Alkaline type will last up to a year and silver oxide type will last 1-2 years but are more expensive.
To access the battery you need to carefully pry off the back cover with a sharp edge (eg. paring knife).
There is a small lip to pry under if you look carefully (I need reading glasses for this).
After the battery has been changed, pop the back cover back on in its original orientation - there is a cutout that goes over the stem.