You can edit metadata for single items quickly and easily in the web interface.
You can export a collection or community's metadata and edit items in bulk, then reimport it.
If you're making a lot of changes, you may have to split the .csv into batches and repeat the above steps. DSpace's limit here is the number of changes made rather than the number of items you're importing; if you have few changes you can have larger batches, and vice versa.
If you need to create a new metadata field: