How do I restore an app I accidentally deleted on my iPhone?

See here:

Redownload apps and games from the App Store - Apple Support

start app store

click on your portrait in the upper right corner

click on apps

click on my appps

click on not on this iphone

find the deleted app on the list and reinstall it

Assuming it's still available from the App Store, you should be able to download it again; you won't be charged to do this whether or not it was a paid app in the first place.