In the Activities tab, you can select multiple Activities and assign them all a new Due Date, which will be the same for all.
However, is it possible to bulk postpone them?
For instance, let's say you have:
Activity A - Due Date: 03/04/20
Activity B - Due Date: 17/06/20
Activity C - Due Date: 20/12/20
And you bulk postpone them by 1 month, so they become:
Activity A - Due Date: 03/05/20
Activity B - Due Date: 17/07/20
Activity C - Due Date: 20/01/21