If you've changed to a new mobile phone, you can transfer chat history from your old mobile phone to your new device as the following method:

Log in to your WeChat on your original mobile phone, tap Me > Settings > General > Backup & Migrate Chats > Migrate Chats to Another Device > Migrate Entire Chat History / Migrate Partial Chat History, select the chats which you want to migrate and then follow the instructions and operate on your new device.