Whags the best way to download an old version of an app on an IPad?

[Mandarin Duck]

I recently updated an app on my iPad and I made a huge mistake because the new update is complete trash
How do I take off this new version and put the old version back on?

I did see that if I had an older version on my iTunes I could do it that way, but I didn't backup the older version so I can't do that

 

[Medulla Oblongata]

What app?