If you have a big database (many post with many photos), i recommend you should to backup data before to do.
How to update a theme:
1 – Go to Plugins –> Add new –> Search for “Easy Theme and Plugin Upgrades“
2– Click install Now
3- Click Activate Plugin.
4– After installed plugin, navigate to Appearance > Themes. Click the “Add New” button at the top of the page.
5– Click the “Upload Theme” button at the top of the page.
Click the file browse button to select your theme zip file. The button’s text varies by browser. It typically says “Browse…“, “Choose File“, or “Choose…”.