Ah yes- the ceremonial first battery pack replacement. Brings nostalgic tears to my eyes
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As previously stated, simply unscrewing the cover of the battery compartment(s) will get the job done. Note that on both of the narrow ends there is a convenient slot into which you can poke a straight bladed screwdriver to help pry the tightly O ring sealed battery compartment cover off.
Keep in mind that, in the future, you have other battery pack options. My personal preference is for the 2 C cell lithium battery pack w/ 8 ~ 9 AH rating that will last most folks upwards of 2 years. A 9 v lithium single cell has a 1200 mah rating and works for the APECS 2.0x electronics as well. A simple harness that connects a standard 9 v battery clip to your spent pack’s Molex connector does that trick. Get the polarities correct or you will be sorely (and expen$ively) surprised. Another option is your basic plastic 2 AA cell holder filled w/ 2 lithium 3.6 v AA cells. Their 2400 mah rating more or less duplicates the 5 AA alkaline cells and is far more resistant to cold temperatures. Again, wiring the Molex connector from your spent battery to the clip does the trick and again, same smoke test caveat applies.
Have fun!
Ken