To be safe wait for tomorrow's rain and let it get rained on. From the photo, the upper leaves look upright/firm, if it needs water the leaves get droopy, so the roots are still getting some moisture from the wet soil. The upper part (2 inches above) that is dry is not where the roots are located.
Don't worry about the yellowing of the older leaves. It is normal for plumerias to shed the older leaves. In fact those yellow leaves will probably fall off easy if you touch them slightly, and if they fall, observe if you get more yellow leaves now that you are having it enjoy the sun and not indoors.
Tomorrow your time, let your plumeria enjoy the rain.
