Is it true that when you kiss, and you get your friend's saliva in your mouth and even allow it getting it into your body, your dna will change?

Nothing to worry about, saliva cannot change DNA.

Regular infections, such as mononucleosis can be transmitted through kissing, but the host’s DNA is not affected from exposure to non self DNA, which is rapidly disintegrated.

Your DNA will not change from kissing or even having sex. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV and Covid vaccines. Wish you good health!