How to train from 224 to 518?

[Jokerjokerspy]

I'm training a custom dataset of CXR myself.My dataset is about 800k images.If i use 518 for the whole training it will take up too much time ,so i decide to train in 224 first .I am just wandering how to change the resolution from 224 to 518 during training as mentioned in the article.Should i resume training for some more iters or just fine tune from 224?And how many iters would be proper ?How should i modify the learning rate ? @qasfb thank you for reply in advance.