To remove the board there are four plastic stand up posts. You need to have a pair of needle nose pliers and remove them one at a time. There are also two black screws over the BRs. To remove the entire transformer and board - There are three black screws on the RHS - facing you and three behind the transformer. Make sure the machine is switched off when working in this area - in fact, remove the mains plug entirely.
The process of replacing the BR is covered in detail here ;
http://www.pinrepair.com/bally/index1.htm#ps
