I need help with deleting keyframes which are on armature. So I had one project file in which I had a walk cycle and then I duplicated this project to have a run cycle but it has the old keyframes... How do I delete them?
Many ways to do this: keyframes are stored into a so-called "Action".
If you turn a window into a dopesheet window you will see all your keyframes, that you can select and delete with the X key.
But if you turn the dopesheet editor into an action editor you can animate a new action without deleting the old one, simply don't forget to press the "f" sign near its name (f for Fake User): this operation ensure that your old action will not be deleted even if it's not the active (used) action. Then press the little x sign to close the current action and create a new one.
After you have animated the new action, you can select the action to perform with the action editor menu (or even combine them with the NLA editor).