Press this button, you will get your SDK manager, you will get screen like this,
Just check mark that update and click on Install package and follow instruction.
Edit
Or may be you or someone have deleted your SDK folder by mistaken, in this case you must have to download all the SDk's which is required. Or you can copy from some one else and paste it somewhere in your system and just browse the path. That is also one way, so you don't have download it otherwise download it