Starting with POA 6.0, versions 1 and 2 skins (created before POA 5.4) are no longer supported.
New version 3 skin must be created for each skin of an earlier version. There is no automatic upgrade procedure.
WARNING: New skins must be applied to brands before the 6.0 upgrade, otherwise these brands will get the default skin.
POA skin customization documentation is available here.
You may also find the table in chapter 5 of POA 5.4 skin customization guide useful.
Once the skin is ready, upload it to the APS catalog as described in documentation, it will be validated and approved for using in your PA environment. Validation typically takes one or two business days.
After that it can be uploaded into POA and applied to brands.