Hitting the Prioritise button on the Issues Page of the RAIDD main tab will sort the open and accepted Issues into a priority order of High, Medium, Low (or 5,4,3,2,1) Severity. This is based on the Severity Rating (or Residual Severity Rating if Extended Risk/Issue Features are enabled) 
Note: Ensure that the Issues screen is visible by checking the correct radio button on top left of the screen. You will need to have either split view or issues selected.