If you are building your site all you need to do is follow the WA website builder promps.
First you will click the "Build My Website" tab and you will be taken through the building process.
You will need to pick one of the pre loaded themes at initial build of your website, then you will be free to change the theme at will.
If the theme you want is not one of the supplied themes pre loaded onto the builder, just use any theme you want as it will be temporary. If your theme is already pre loaded, after you finish this process you will all done. :)
For those of you who did not find the theme you want to use pre loaded and you have a copy of it downloaded on your computer, or for those who are just wanting to change your theme, lets go to the next step. How to change your theme.
How to change your theme:
The first thing we need to do is make sure you have a back up of your site.
Do not proceed any farther until you have a backup.
You can find training for this here:
https://my.wealthyaffiliate.com/training/how-to-back-up-your-wordpress-site-using-tools-export
This is only one of many ways to back up your site, but I like this way best myself.
What I did, was to slowly go through many of them, then investigating each one that I liked, then checking to see if was compatible with smart phones, ipads, tablets and the like...right from the get-go.
With so many new gadgets coming out, a person should have a theme that will adjust all by itself.
Sure glad I did that right from the start. Saves a lot of grief later on.
:) Tom