To change speed on a clip, right-click it and make that choice.
Changing speed for an entire sequence might best be done after first exporting the sequence without the speed change. Makes for less demand on the CPU/GPU.
Once exported, bring what will be a single clip back into VP. Apply the speed change and export again.
An alternative method would be to select ALL sequence content, right-click and Change Speed.
In any case, at each step, Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar. Then you're protected, if something goes haywire.