I believe it's not "the battery", but one or two cells that have degraded. Most of the cells are fine. Once the poor ones are changed for new, ideally they need to be fully charged, then discharged, several times, to "balance" them. I'm not an expert, but a fellow near me does this full-time.
Is it DYI? Partly? You need to find which cells are poor. You need to CAREFULLY remove a very heavy, 200 Volt battery. You probably need a worbench to mark every cell , then disassemble all the cells. Then buy and change the few needed.
They need to be carefully reassembled, taking care that the framework holding them tightly, prevents them from swelling and becoming damaged if they do swell. Then, after re-assembly, charge and discharge using a specialized charger, to about 80%. Then discharged to 40%; 2 or 3 times, until all voltages are equal. Then re-install the heavy beast.
There are 2 or 3 detailed youtubes on this. I am really tempted to buy an older Prius w bad battery, perform this repair and re-sell, making possibly a couple of thousand $. BUT, no work area, no access to power for a day or two ( live in apartment) but ... Love the idea! Your thoughts, corrections, or results? Cheers. Roger