my apex thermometer reads several degrees low, how do I calibrate the thermometer?

Here are the directions for calibrating: 

"The thermometer is initially calibrated at the time of manufacture. lf the thermometer is used according
to the use instruction, periodic readjustment is not required. However, we recommend checking
calibration every two years or whenever clinical accuracy of the thermometer is in question. Turn on the
thermometer and insert into the water bath which the temperature is steady. ASTM laboratory accuracy
requirements in the display range of 98.6 to 102.2°F (37.0 to 39.0°C) for electronic thermometers is
±0.2°F(±0.1°C). The above recommendations do not supersede the legal requirements. The user must
always comply with legal requirements for the control of the measurement, functionality, and accuracy
of the device which are required by the scope of relevant laws, directives or ordinances where the device
is used."

What size batteries does this take?

This particular thermometer uses a 1.5 V DC button battery (size LR41, SR41, UCC 392, or equivalent).