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Pull off the shrink wrap and take some photos. They are 2x 3S2P packs = 10amp total. The stock cells are 2600 mAh Sanyos. The two packs are wired in parallel, with both feeding the backside of those banana plug fittings. There will be a small plastic insulator under the disk preventing the cells from contacting the backsides of the banana plugs.
I recommend using an extra layer of insulation around the +ve terminal of each cell to avoid the slight risk of the nickel strip connecting the +ve terminal to the case which will spark and/or start a fire. You should be able to find the stick on insulating donuts from the same places you get cells. They are like 20 for $0.80 here and worth the extra peace of mind.
Most folks use a hot glue gun to keep the cells oriented and aligned while you spot weld. Spot weld up the 6 new cells into the same +/- ve orientation.
You can desolder the existing BMS wires, they will be connected to the nickel strips. Be careful to not get the insulation under the nickel strip too hot or it will melt the donut insulator or the case wrap and short out the cell.
In a 11.1V 3s nominal pack there should be 6 wires total:
Pack positive (cell 3) to BMS
Cell 1
Cell 2
Pack negative to BMS
Battery out positive to banana plug
Battery out negative to banana plug
Resolder the wires to the new cells again being careful to not melt the insulation under the nickle strip with too much heat. A 30w soldering iron, a clean tip, and low temp flux core electronics solder.
You can wrap the whole pack in electrical tape for structural integrity if you don't have shrink wrap. Put some plastic, cardboard or something alongside the packs in the canister so they don't rattle.
Reinstall the plastic insulator between the banana plugs and the pack
Push the disk back into place evenly and use a dental pick to put the locking o-ring back in on top of it.
