We need some more details about your saber and the battery pack you looked at to give an exact answer (look at your order in case it's a customized saber and not a Grab Bag/Mystery Box/Raffle Prize):
1. How many batteries are in the battery pack?
2. Has it sound?
3. If it has sound, what kind of? Obsidian Premium (v3/v4) or Obsidian Lite?
4. Has it an additional board - the Emerald Driver or the Diamond Controller?
If you can answer 1. with "4" and 2. with "No", then you have the 4xAAA battery setup. Those need to be replaced once they're drained. You can use either one-time batteries or rechargeable batteries of the AAA size as replacements, but don't mix them.
If you answer 1. with "2", it's the Li-Ion setup - see below.
If you can answer 1. with "4" and 2. with "Yes" and "Obsidian Lite", it's the 4xAAA setup again.
If you can answer 2. with "Yes" and Obsidian Premium" and/or 3. with "Yes" (Diamond makes question 2 redundant), then you have the Li-Ion setup again.
Li-Ion setups are handled like this:
NO replacing with normal AA batteries! Always replace with 14500 PCB protected Li-Ion batteries!
NO charging with regular battery chargers! Always charge with a special Li-Ion battery charger, for example the VC2 from XTAR!
You may also post a picture of the battery pack pulled out of the saber if you're not sure how the boards look like and can't answer the respective questions
Replacing is simple - pop out the old batteries, put them in a Li-Ion battery charger (if Li-Ion) or throw them in a separate waste (if AAA - don't throw in them in the regular waste as they need to be disposed of properly due to harmful chemicals in there!).