how to delete app purchase history

You can't permanently delete them from your account's purchase history, all you can do is hide them from the cloud via your computer's iTunes : Hide and unhide purchases in the iTunes Store - Apple Support (there is a bug on iTunes which means that when you hover your mouse pointer over an app the 'x' may not show on its top left corner, but clicking there should still hide the app)

On your computer's iTunes, go to the Purchased section - either via the Purchased link under Quick Links on the right-hand side of the iTunes store homepage, or on the current version of iTunes by clicking on your name towards the top right of iTunes :

Select the Apps section in Purchased and click on the top left corner of an app that you want to hide (theoretically an 'x' should show on that corner when you hover your mouse pointer over the app, but a bug seems to be stopping it from showing). The app should then disappear from that list, and from the Purchased tab in the App Store app on your phone.

You can unhide apps via your account on your computer's iTunes : log in via the Account > View Account (Store > View Account on older iTunes) menu options, or 'Account Info' as shown in my above screenshot, and on your account's page there should be a 'hidden purchases' section (the section will only show when there are items hidden).