You can use two methods to update resources: PATCH and PUT. PATCH is meant for only partially updating a resource, whereas PUT is meant for entirely replacing a resource with a new one.
When replacing a resource with PUT, include all the resource's fields in your request, not just the ones you've changed. Your new model of the resource will completely replace the current model, so none of the old fields will carry over to the updated resource unless you include them in your request data.
Take a look at the individual documentation pages for PATCH and PUT to see how to use them to update resources.