What should i do if she trys to kiss me?

so this girl and i have known eachother for years through our youth group and have always shared a connection. however recently she has been wanting me to kiss her just to see what it's like to be with me. i dont know how much she wants but we are both in relationships and i do not want to hurt my girlfriend by kissing this girl if i shouldnt. so that is my question. what should i do if she tries to kiss me? is that considered cheating to kiss someone once and we have known eachother for years? i just need to know