You just have to download the App iHealth MyVitals on your new smartphone and login your iHealth account.
You will find all your previous data and you could sync new measurements.
*credit to Sylvain of iHealth Labs EU
https://helpcenter.ihealthlabs.eu/hc/en-gb/articles/211479529-iHealth-Core-HS6-I-change-my-smartphone-what-should-I-do-