HP 15c - Manual (Page 260)

260 Appendix F: Batteries
To install new batteries, use the following procedure:
1. With the calculator turned off, slide the battery cover off.
2. Remove the old battery.
3. Insert a new CR2032 lithium battery, making sure that the positive
sign (+) is facing outward.
4. Remove and insert the other battery as in steps 2 through 3. Make sure
that the positive sign (+) on each battery is facing outward.
5. Replace the battery cover.
Note: Be careful not to press any keys while the battery is
out of the calculator. If you do so, the contents of Continuous
Memory may be lost and keyboard control may be lost (that
is, the calculator may not respond to keystrokes).
6. Press = to turn on the power. If for any reason Continuous Memory
has been reset (that is, if its contents have been lost), the display will
show
Pr Error . Pressing any key will clear this message.