Having an app (even one you build) that uses the API will let you change a value of a metafield. Take a look over the docs and see if that is aligned with what you want.
If it's not, what is the use case you're trying to build for?
★ I jump on these forums in my free time to help and share some insights. Not looking to be hired, and not looking for work. http://freakdesign.com.au ★
