We don't offer a feature to edit multiple activities at once - although we know it's a popular request. For example, after bulk importing historical data you may wish to bulk apply activity settings such as gear, privacy, activity type, tag, etc.
For now, you can most efficiently edit activities one-by-one on the website from the My Activities page. You can select Edit, adjust the activity settings, and Save for each activity without having to leave this page. You can also filter activities by activity type, keyword, commute or private tags.