My Garmin is adding on more than 2km every time I run if I run 12km it’s telling me I’m running 14km how do I fix this

It sounds like an odd behaviour. Could you post a web link to an activity and make it public so we can have a look at it?

How do you know the correct distance?

Ok. I checked now and see that it doesn't have a GPS or can't use the GPS in the phone. Then it uses your step length and steps to calculate the distance. Check your step length and adjust if needed.