The outcome of changes to delays in your workflow will depend on how you are changing the delay:If you change the timing of the workflow delay to execute later (greater delay), the contact will be rescheduled to adhere to the new delay. For example, contact A is scheduled for a 1 day delay, then you change the delay to 2 days. The contact will not be rescheduled immediately, but will wait the original 1 day delay. At the end of that 1 day delay, the workflow will see that the delay was changed to 2 days and the contact will be rescheduled for the action based on the new delay (they'll wait 1 more day for a total of 2 days because they already waited 1 day).
If you change the timing of the step to execute sooner (less of a delay), the contact will adhere to the original delay. For example, contact A is scheduled for a 2 day delay, then you change the delay to 1 day. The contact will still move on to the next action 2 days after the start of the delay.
To learn about what will happens to contacts in your workflow when you delete a delay, check out this article.