Hello before to update theme, Please make sure you have not done any changes in theme core file. If you have done any changes in main theme like (Update or change css, update or change language, Any function) Please save all change before to update theme. 

But if you are using Child theme then no need to save changes from Child theme because you are going to update only Parent Theme File.

Important : Never update child theme file.

Step 1: Download Latest File from themeforest.

Step 2: Extract Zip file and then you will see one an other theme file like (classiera.zip, themename.zip), Extract this file also.

Step 3: Login your FTP and go to root/wp-content/themes/

(Note: If you are using Classiera Version 1.7 or older version then first go to Plugin Folder via FTP. 

root/wp-content/plugins/ you will see one folder by name of "joinwebs-locations" delete this folder)

Step 4: Now Just upload(replace) latest file which you have extract . It will ask you "Are you sure you want to replace all files?" Just Click on yes.

After to upload complete file you will see your have updated version.

Very Important: If you have already translate your .po (language) file , Then before to update theme please save (download) your language file and after to update theme file just replace this file and it will be work fine. Or Just replace .PO file and never replace .MO file.

These things will be never lost while updating theme (All Post, All Categories, All Theme Settings), So don't worry about these.

Thanks

Regards
JoinWebs