I'm afraid you'll need a Wii U to do that.

If you don't have a wiiu you can use CEMU as a replacement. First download a backup of the miiverse app then import it to cemu. After go to CEMU settings go to online tab then press the Pretendo checkbox, you should put your Pretendo backup profile on there.

P.2 To download your pretendo profile data file go to pretendo website then import it to CEMU. After that open the miiverse backup on cemu will launch the app then you should be connected to pretendo servers as wiiu user, now there YOU CAN MANAGE AND DELETE POSTS.

For more information visit Pretendo website or discord server.