In order to change due dates of multiple tasks that don't have recurring dates, please use the multi-select feature. Press and hold the CTRL key and while holding it, click on tasks names to select them. Once you select the first task, you'll see a menu appear at the top with options that can be performed on all selected tasks at once.
Thanks for the answer!
Before I posted the question I found the info in the General Shortcuts but couldn't get it to work. The missing step there is as you mentioned "Once you select the first task, you'll see a menu appear at the top with options that can be performed". Jim
Is there a way to do multi-select via the web on a tablet?
I'm afraid that might not be possible. This feature is only triggered by clicking on he tasks while holding CTRL.
Nonetheless, thank you for your feedback, we appreciate it and will consider finding a solution for this issue in our mobile apps.