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On the back of the shaver, you should see 2 screws. After removing them, you'll have to pry the 2 halves of the case apart. Be careful, because there is a rubber o-ring type gasket that you don't want to damage.

Do you already have a replacement battery?

If not, the battery model is WES8026L2508

You may be able to find them online, however this part has been discontinued, and even when available, is typically available to only authorized panasonic service centers. This is because it's not a simple plug and play replacement, you have to physically remove the old battery and solder a new one into place.

When factoring in the replacement cost of the battery, usually the need for new o-ring and seals after cracking the case, and the time involved in the replacement process, it's typically more economical to replace the entire shaver, rather than try to have it serviced.