How to change keywords for just one ad in an ad group
If you want a specific ad to be triggered for one or more keywords, you need to put those into a separate Ad Group by themselves. (That's what an "Ad Group" is--a group of keywords you want served with the ad or ads you include in the group.)
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Hello Connie L ! Welcome to the Community !
When you change a keyword in an ad group, it affects all the ads in that ad group. All keywords from an ad group can trigger all ads from that ad group to be displayed.
If you want only one keyword to be assigned to an Ad , you have to create an Ad Group with that 1 ad and that 1 keyword.