HP 33s User Manual page 297

Once you've removed the batteries, replace them within 2 minutes to avoid losing
stored information. (Have the new batteries readily at hand before you open the
battery compartment.)
To install batteries:
1. Have two fresh button–cell batteries at hand. Avoid touching the battery
terminals — handle batteries only by their edges.
2. Make sure the calculator is OFF. Do not press ON (
the entire battery–changing procedure is completed. If the
calculator is ON when the batteries are removed, the
contents of Continuous Memory will be erased.
3. Turn the calculator over and slide off the battery cover.
4. Never remove two old batteries at the same time, to prevent
memory lose . Remove one of the two batteries. Press down the holder. Push
the plate in the shown direction and lift it.
Support, Batteries, and Service
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) again until
A–3