On the pull-down menu for your profile (it's next to My WikiTree and labelled Vogan-18), click on DNA. Then click on "DNA Tests" (It's the middle box of the 5 underneath the heading). Scroll down until you see the panel for AncestryDNA and click on Edit or Enter more details.
If you added your MtDNA to any of the other vendors (23andme, FtDNA or Other) use Delete instead of Edit.
Don't forget to Save the Details for each test you change.