Problem
You would like to adjust the previously defined project period in the process plan afterwards, for example, have the project start earlier.
Solution
The 'time scale' is always defined by the minimum (earliest process in your process plan) and the maximum (latest end of a process). These limits remain, even if you delete the corresponding process later.
Increase time scale to the left / start project earlier
You can set processes to any date simply by changing the start date in the process details on the right side in the tool.
Increase time scale to the right / let project end later
You can change processes at any time by drag& drop or by changing the process duration in the process details on the right side of the tool.