hey @Lanc3! welcome to the community 
if you want a mutation for only that field, what you’ll need to do is as follows:
on the api side:
- define the mutation type to accept whatever parameters are needed to update that field, ie the post id and field value
- create the corresponding mutation resolver in the model’s service
on the web side:
- create a form that uses this mutation
reread chapter 3 for more detailed examples and information: Saving Data | RedwoodJS Docs