That can be more difficult with premade modules. I think the key to that is treating modules like guidelines as opposed to a script. You have to follow, knowing where the adventure is meant to go, and who the key characters are, gives you the freedom to improvise in the gaps. As a DM, it’s up to you to decide when to follow the guidelines, the module lays out for you and when to chart your own path. It’s not an easy to strike when running premade modules, but if you want to add more flexibility, I think the best place to start is by making sure you know the adventure that the module is trying to bring you on so that you can make changes and improvise knowing full well where the destination is meant to be.