It doesn't appear that the specific 'ask' has been answered, so for completeness...
Many products such as battery packs with 'no serviceable parts inside' (as opposed to no ''user'' serviceable parts) are not even glued together, they use a sonic heat process to melt the plastic at the edges in a permanent 'weld' process. They do this primarily for speed and cost, but I think they also appreciate the side benefit that assemblies are impossible to disassemble without leaving visible marks. Visible marks indemnify the manufacturer in case of disaster or injury during use following modification.
How to disassemble? Depending on the product and your skill, use your own imagination. I would probably use a dremel tool with a thin blade and attempt to follow plastic seams. Obviously care is needed to avoid cutting through the battery cells' metal container. It would be best to cut at an angle or perhaps halfway across a seam at one angle and a different angle for the other half so that the angles help with realignment upon reassembly. For reassembly, use glue and fill the saw gap. Hot glue would possible work well. File or sand any aberrations.
Another respondent described (did not necessarily recommend) the practice of replacing the single bad cell in a series-pack. So, it should be noted that li-on batteries have limits on the number of charge-discharge cycles that they are capable of; they do wear out. If not replacing all cells (regardless of whether they presently work) one would be limiting the life of the repair to the remaining life of the not-new cells. A person's time and effort may be worth more than the cost of replacing all batteries in one shot.
While you're at it, build a back-up pack...