What device are you playing on? If Windows — just delete the old version, download and unpack the new one. Saves are stored outside the game folder (in AppData), so they'll remain and will be found by the new version. If Android — don't delete the old version, download the new one and run the installer, then choose the "update" option.
The new version mainly includes Journey mode expansion, voiceover, updates to some older scenes, and various bug fixes.