Hi PE,
It depends.
When all tasks have been reset to 0% done, and there are no tasks with
constraints, it is enough to change the project start date:
Project - Project Information - Project Start date.
When you DO have constraints in the project you will need to run
Adjust_Dates.
Up till Project 2000 this was delivered as a macro (Tools, Macro,Macros...)
In more recent versions it is an add-on available through the Analysis
toolbar (Tools,Customize, toolbars, etc.)
And when there are tasks that are NOT 0% finished you should FIRST set them
to 0% finished.
HTH