omg, thanks for asking this, cause i totally am having the same exact issue! haha, silly.
Swiffer Wet Jet Change Batteries
Pull the cleaning bottle. It is behind it in a panel with a finger hole that says "open". No tools required. Four AA bateries.
Just below & fixed to the handle there should be a small flat casing that’s where the batteries go, underneath is where you pour the detergent, the casing holds 4 AA batteries,
try under where the cleaning solution goes…if not check their website maybe there are instructions. im not sure.
The panel is on the rear side of the handle, as Trekkie says-below the detergent bottle. The panel is wider at the top than the bottom.