4.5 by 6.12 original drawing want to increase size to 18 by 24. How do I do it?

24/18 = 4/3

6.12/4.5 = 1.36

That's pretty close to the same ratio, so just scale up by

18/4.5 = 4

Just multiply each dimension by 4.
That will produce a size of 18 by 24.48