Apple Watch does not offer an option for configuring the frequency of heart rate measurements.
Heart rate readings can be taken manually via the Heart rate app:
Check your heart rate
Background heart rate readings are taken automatically at intervals that will vary, depending on your activity level, as explained in the following support article. Readings could, for example, be taken automatically as frequently as every one, two or three minutes:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Heart rate is also automatically recorded continuously during workouts when recording them via the built-in Workout app and via third-party fitness tracking apps that support the feature:
Work out with your Apple Watch - Apple Support
If you would like to submit a feature request to Apple for any other options, then you can do so here:
Feedback - Apple Watch - Apple
Various third-party apps also offer heart rate-related features. To browse and search for apps with the results pre-filtered to include only those iPhone apps that also include an app for Apple Watch, use the Watch app on your iPhone. Apps can be searched by keyword or app name via the Search tab.
Use apps on your Apple Watch - Apple Support