Hello Moderas,
Thanks for using Apple Support Communities.
I see that you are going to be trading in your iPhone, and you want to try and clean the sticker marking that has been left on the back. I completely understand wanting to have your iPhone in great condition when you trade it in. I'm happy to assist you with this.
Use the article Cleaning your Apple products, specifically, these sections:
The materials used to make Apple products vary; in some cases, each product might have specific cleaning requirements which vary by the part you're cleaning. To get you started, here are some tips that apply to all products:
- Use only a soft, lint-free cloth. Avoid abrasive cloths, towels, paper towels, and similar items that might cause damage.
- Unplug all external power sources, devices, and cables.
- Keep liquids away from the product.
- Don't get moisture into any openings.
- Don't use aerosol sprays, solvents, or abrasives.
- Don't spray cleaners directly onto the item.
If liquid does make its way inside your Apple product, get help from an Apple Authorized Service Provider or Apple Retail Store as soon as possible. Liquid damage isn't covered under the Apple product warranty or AppleCare Protection Plans. If you plan to visit an Apple Retail store, make a reservation at the Genius Bar (available only in some countries).
iPhone 5s, iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus
Clean iPhone immediately if it comes into contact with contaminants that might cause stains, such as ink, dyes, makeup, dirt, food, oils, and lotions. To clean iPhone, unplug all cables and turn it off. Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, compressed air, aerosol sprays, solvents, ammonia, or abrasives to clean iPhone. The front glass surfaces have an oleophobic coating. To remove fingerprints, wipe these surfaces with a soft, lint-free cloth. The ability of this coating to repel oil will diminish over time with normal use, and rubbing the screen with an abrasive material will further diminish its effect and might scratch the glass.
To clean the Home button (allowing Touch ID to work optimally), wipe it with a soft lint-free cloth.
Have an amazing day!