Update theme via WP admin

I must warn that this is probably a very elementary question based on my basic lack of understanding. I am confused about the instructions provided in the documentation for my theme, AIT Directory Portal, regarding updating manually via WP Admin.

The instructions read:

“1. Deactivate currently used theme – activate another theme, for example one of default WordPress themes.”

“2. Delete theme which you are going to update. Open theme Details and click Delete button.”

My confusion: My currently used theme is an older version of the AIT Directory Portal. If I deactivate and delete my theme, will I lose all information contained on my website which I built with that theme? Or will that information reappear once I upload the updated theme via the next instructions:

“3. Click Add New, on the next screen click Upload Theme button and browse for downloaded zip file with theme. Click Install Now.
4. Theme is successfully installed, click on Activate.”

Is there anything I must do after activating the newly installed up-to-date theme, in order fill it with my previous content? or does that happen automatically?

Thanks for any help,
Andrew