File -> Update Data (Ctrl U)

File->Update data reloads all bounding boxes in the current data layer from the server.

You should run File->Update data if you want to refresh the data in the current data layer with the data the server currently holds. JOSM won't overwrite local data, but it may detect, that newest object versions from the server collide with objects you are working with locally. In this case, JOSM will create conflicts. You will have to resolve them after the update is finished. Your local edits aren't lost.

This action is disabled unless the currently active layer is a data layer.