Actually it is a 2-step process. Download PAF4.0.4 from www.familysearch.org and install it. Open the PAF2 data, which will be converted to PAF3/4 format. Either use it in PAF4 or download and install PAF5.2 (there is no need to un-install PAF4). Open the PAF4 data in PAF5.2, which will convert it to the latest format.
I never did understand why the PAF2 converter was dropped in PAF5, but it was. But there is a solution in any case.