Open any contact, company or contract in the iOS or Android app and tap Change Log, the last tab, to see who changed what and when. Each entry shows the person who made the change (or System for automated changes), how long ago it happened, and every field that changed with its old and new value.
The list loads more history as you scroll. It shows the same history as the Change Log tab on the web. Restoring a previous version is still done on the web.