A flow might be in two modes:
While you are working on your flows, they are in the "Edit" mode. Once you are comfortable with your logic, you can switch the flow from the Edit to Live mode.
When you turn the flow into a Live mode, the Flow Validation triggers to check that your flow components have all been set. If something is missing, the actions and conditions that did not pass validation will highlight in red and the flow will not switch to a Live mode.
When your flow is Live mode, tou can edit some properties, such as change the message subject, but you cannot edit any major parameters, such as timers, actions, etc.
When you switch from Live to Edit mode, all existing timers that will trigger in future will be canceled.
As such, please ensure it is what you want to do.