If the rechargeable batteries in your Philips Philishave HQ6854 die, you have to get new ones immediately or you'll find yourself walking around half-shaven. You can quickly replace the batteries by taking off the back cover of the shaver, plucking the old ones out and inserting the new ones. There are only two screws securing the back cover to the shaver, so you won't go through any difficulties in removing it.
Turn on the Philishave. The battery removal instructions provided by Phillips indicate that you should turn on the shaver before you replace the batteries to ensure that they're completely dead.
Pull the power cable on the Philishave out of its power source.
Disconnect the power cable from the Philishave.
Allow the Philishave to run until the battery dies completely and then turn off the shaver.
Extract the two screws from the bottom of the back of the Philishave and remove the back cover.
Remove the batteries from the Philishave.
Insert the new batteries, following the indications on the shaver to see where the plus and minus ends of the batteries should go.
Assemble the Philishave to complete the procedure.