Changing of theme and data issue?

Hi dear catch themes community.  I am a  newbie in the field of blogging and freelancing. I have a hesco bill site. Which is currently based on another theme. But I like the blakely theme. So wondering that how will I transfer from the previous theme to the blakely theme. What will I do with the content and large number of pictures. Will it be disturb or there is anyway by which I can change the theme without disturbing its data and content.   Thanks