Dismount the Danalock V3 from the door (optional).
Open the battery compartment and pull the two black ribbons to release the batteries.
Replace all four batteries with new ones (never mix used and new ones - see disclaimer below).
With the new batteries in place put the battery lid back on and reattach the Danalock V3 to the door.
Recalibrate the lock and you are good to go again.
You can watch the video about calibration methods.
Disclaimer: It is very important to replace all 4 batteries at the same time. Use only approved disposable CR123 lithium batteries. Click here to find the list of approved batteries. Not replacing all batteries at the same time can cause the batteries to heat up and potentially cause them to explode. Danalock has no responsibility in any way if the replacement of batteries is done inaccurately.
You can read more about safety and disposal and batteries here.