Deleting ancestry site cookies depends on which browser you use.
I use Firefox - https://support.mozilla.org/en-US/kb/delete-cookies-remove-i...
You can find similar instructions for your preferred browser in the help/how-to files for that browser.
It is important to delete all ancestry cookies - they will be numerous. For the sake of your saved passwords etc., it's important to delete only the cookies for ancestry - not "all" cookies.
With all that occurs when we search and save from the ancestry site, cookies can and do become corrupted. I clear my cache several times daily and I delete my cookies at least every couple of weeks.