How do I amend the fixed “author” part in the URL?

Use the plugin Edit Author Slug:

This plugin allows an Admin to change the author slug (a.k.a. – nicename), without having to actually enter the database. You can also change the Author Base (the ‘/author/’ portion of the author URLs). Two new fields will be added to your Dashboard. The “Edit Author Slug” field can be found under Users > Your Profile or Users > Authors & Users (Users > Users in WP 3.0). The “Author Base” field can be found under Settings > Permalinks.