first apply masking tape below the existing decals and mark the start and finish of the overall length, so you will get the new ones back into the
correct position, then using a hairdryer warm up the decal you want to remove
letter by letter and slowly peel them off. They will come off easily, next you need to clean/prepare the area removing all traces of adhesive residue before reapplying the new ones
after you've stuck the new decals, and removed the application tape give them a firm rub all around the edge and don't wash the car for a couple of days