Apple watch 6. How do I change my workout goal back to open goal?

On the workout type tap on the 3 dots top right corner and tap on"Open" then start a workout and when you've finished tap on "End",Scroll down to the bottom and tap on "Done" the watch will remember your last setting and next time you do a workout it should be set to open goal,if not you may have to do it again as the watch enters whatever goal you set most and uses this as a default.

Here is a post from another forum that fixed the problem for this lady;

What I did to fix it was start an "OPEN GOAL" outdoor walk, and walked vigorously for about 1/2 an hour and just hit end when I was done. It still had a 20 min time set in yellow when I was finished. But the next day the 20 minutes had disappeared and OPEN GOAL was back in green, so I am assuming that is how you fix it. It must reset to the last thing you did overnight.

On your watch, click on the ··· next to a workout in the Workout app, and the following screen should show up:

then click Open, and your watch will remember your choice. Hope this helps!