When you run a Split Test, certain settings like your testing variable and ad information can't be modified. However, other settings of your ad group can be edited.
Before you edit a Split Test, you should know that we generally don't recommend editing a split test often after running it, since it will take additional time and budget for the system to relearn. If you edit the control variables of your Split Test, make sure to apply it to both ad groups in order to run a clean A/B test.
To edit a Split Test:
Find the first ad group in the Split Test.
Hover over the name and click Edit. You'll see the Edit Ad Group screen appear.
Make your edits and click Save.
Make those same exact edits to the other ad group in the Split Test.
After making edits, we recommend giving the system an additional 7 days and extra budget to pick a winning ad group.