Simply select the assets you wish to process and click the edit button. Instead of going to the metadata editing view for a single asset, a Multiple Edit dialog will appear:
By default, append mode will be selected, meaning that new metadata will be added to the field content. You may also choose "Overwrite", "Remove" or "Clear field". Overwriting will overwrite any existing metadata by the new field content. Removing will remove existing metadata and Clear field will empty the metadata field entirely.
Edit metadata for a subset of selected assets
While working in the Multiple Edit dialog, you can also update metadata for only a subset of the selected assets without leaving the dialog.
For example, if you have selected multiple assets that share similar metadata, you can update the metadata for most of them at once. If some assets require different values, simply deselect those assets in the selection list. The changes will then only apply to the assets that remain selected.
After applying the update, you can select the remaining assets and update their metadata separately, all while staying in the same dialog.
This makes it easy to handle small variations in metadata without restarting the editing process.