I have recently purchased a frangipani and I see some of the leafs turning yellowish with a dark outline. I was wondering if someone can advise me with what I should do? Any advice would be really appreciated

I will just leave it alone. If it is not spreading, leave it be.
How often are you watering the plant now? If it is getting too hot in your area, it likes daily watering now.

It gets pretty hot here. Every day it goes up to 30-35 Celsius (86-95F). I always check the first few centimetres of soil to be dry before watering it and that happens usually every second day. Do you believe I should water it daily?

That is about how our temperatures goes here everyday too, and then goes higher beyond that to triple digits on for several days. So I water everyday, even doing a second pass after about an hour, since our humidity is so low. I do not even bother to check the soil's dry out time, my Plumies need more moisture by this time.

Plumies can take the heat very well, but it needs good moisture by then, it is after all a tropical succulent. I would do stricter watering intervals if temps are lower than 60F, but higher than that, let it drink as much as it can, and make sure your container has drain holes.

Thank you so much for taking your time and giving me advice. I have 1 more question and I promise I will stop pestering you. I am very close to the sea so the humidity is high in this region (Limassol, Cyprus). I will start watering the plant every day from now on. How much water should I put? Currently I put around 3 litres of water with a single watering is this too much or too little?

Oh that is nice you are near the sea, so there is good moisture in the air. Just water till you see the water drain out of the drain holes. As long as your soil mix is gritty and well draining, Plumie will be okay. Plumerias when awake are quite high humidity and very thirsty trees. Only when they start to go dormant as the cold season approaches do we gradually lessen watering, to no watering at all when winter conditions arrive. I actually enjoy it when they rest, one less plant to worry about in winter.

Don't worry, we enjoy answering questions, since we also learn mutually as it goes. Good luck on your plant!