How do I change my outdoor walk to an indoor walk

Does it not just show as "Walking"?

You can add data to Health app but the only option data type is "Walking"

You can delete data in the same way.

Manage Health data on your iPhone, iPod touch, or Apple Watch - Apple Support

Delete the workout and enter a new one;

A workout can be added manually to the health app,firstly delete the incorrect workout;

Open the Apple Health app,tap on browse,then tap on activity,then tap on workouts,scroll to the bottom and tap show all data

Swipe left on the workout you want to delete and tap “Delete.”

The workout will be deleted, and the changes will sync back to your Apple Watch.

And in the fitness app tap on Workouts/Show More,then swipe left on the workout and tap delete.

Then;

In the Health App tap on the browse tab at the

bottom then tap activity and scroll down to workouts

then tap on workouts and in the the top right hand

corner of the screen it says add data,tap on that and

you can enter a new workout and when you're finished tap on add

at the top right of the screen.

Be aware though that this only effects your 3 rings on the day in question as it cannot be added retrospectively.