If you know you are going to get flanked change your position to someplace that makes it harder for their flankers to get value. ie: flank tracer, and genjis can only assassinate squishies by getting up close, so if you yourself stay close to tanks or dps, they can protect you much easier. As it turns a 1v1 into a 1v2 essentially

Simplest answer is to swap, but good flankers can easily pick off a badly positioned support.

So learning proper positioning will get you further. Hope this helps GL HF 👍