How do you merge layers in Umap Openstreet map ?

Hello, I have created a map using umpa with OpenStreet Map where I placed features one by one. I assigned these features to three different layers. Let's call them layers A, B and C. Now, I have decided that layer C is not a useful category and I want to merge it with layer B. Can I do this without changing the layer of each feature individually? Can I merge the two layers? Or can I perhaps change the layer assigned to a bunch of features at the same time? There are dozens of features in layer C and I would hate to have to change their layer one by one. Any ideas? Thanks!