If you want to downgrade iPhone or iPad application version for some reason you can do it in Windows with iTunes with AppStore support (in 12.7 Apple removes access from iTunes to AppStore, you can use 12.6.3 x86 or x64) and fiddler free tool from Telerik official site [link].
First, install fiddler and install DO_NOT_TRUST_FiddlerRoot certificate to decrypt traffic (Tools > Fiddler Options > HTTPS > Decrypt HTTPS traffic checkbox).
Don’t close fiddler. Make sure that capturing is started (File > Capture Traffic or left bottom corner image) .
Next, start iTunes and find application you need to downgrade in AppStore and download it (I will use for example MyPocket application). Pres download, go fiddler (Ctrl+Alt+F) and find row with ###-buy.itunes.apple.com host name (where ### some symbols, p33 in my example). After this you should stop and remove downloading job in iTunes.
Select this row and enter Inspectors > TextView in right side.
Then click “Response is encoded and may require decoding before inspection. Click here to transform”.
Select “Raw” and scroll down to <array><integer> section:
<integer>822720528</integer> -this version I will use to download <integer>823460947</integer> <integer>823630944</integer> <integer>823655090</integer> <integer>823681567</integer> <integer>823743177</integer> <integer>823809606</integer> </array>
Last integer corresponds to current application version. Count back version from AppStore version history and copy corresponding version integer. Right click row and select “Unlock For Editing”.
Then past integer to <dict><string> section in Inspectors > Raw:
Right click to row Replay > Reissue Request. Find new ###-buy.itunes.apple.com row and make sure that requested old version (can see it in raw section <key>bundleShortVersionString</key><string>4.1</string> when current version is 4.2.5)
Enter bpu ###-buy.itunes.apple.com in QuickExec (Alt+q or black left bottom box).
This action will capture request that we should change. Go to iTunes and download application again. Fiddler will popup. Go to fiddler and press “Run to Completion” first time.
In the second breakpoint hit modify string for integer corresponding application version you need. Press “Run to Completion” again.
iTunes will download old version of application.