Hi @tomenews42 ,
Here’s the [https://thinkpads.com/support/hmm/hmm_pdf/t540p_w540_hmm_en_sp40a26003_01.pdf|hardware maintenance manual] for the laptop.
Go to the bottom of p.75 to view the necessary pre-requisite steps and then the procedure to replace the power button board.
To find a replacement board search online for ''04X5553'' to get results for suppliers that suit you best. (A W540 is the same as the T540p as far as the board goes)