Hi Peritwinkle,
We appreciate you reaching out to us. We understand that you are unable to update your device to iOS 13 or newer.
We also understand that you've already gone through the steps in the article below, and you are getting a message that you are already using the latest update.
Update your iPhone, iPad, or iPod touch
Based on your post, it seems like you are using an iPhone 6 Plus.
If that is correct, you are already using the latest update for your device.
The article below contains the list of devices that are compatible with iOS 13 and newer.
iOS 14
You may wan to consider asking the app developer of your preferred health-related app if they are planning to support older iOS versions.
You can use the article below to help you in reaching out to them.
How to contact an app developer
Take care!