Remove a single screw from the controls board. (It is not necessary to release the small ribbon from its socket on the other side.)

With a spudger, the controls board can now be lifted off the pegs which retain it within the top assembly.

On the hidden side of the board is a small rechargeable lithium cell which maintains the time and settings when the main battery has been removed. Measure its voltage.

If the voltage reads less than 2V it may be permanently damaged and will be unable to maintain the time or settings across a main battery change. It is very important to replace it only by another rechargeable lithium cell, not a regular lithium button cell, which would probably leak and cause damage.

On reassembly you may have to rotate the function wheel until it engages with the function switch on the board.