Ok, I won't lie but there is a lot of work to do. You may be better off starting your new project from scratch using your office template and transferring across anything you need using the Transfer Project Standards tool.
However if you want to know what you need to do to reuse an existing project then do as follows:Detach and save as the original model. Delete everything 3D in the model (unless there are parts you want to keep).
Change project information parameters.
Delete any drafting views you don't want.
Remove all existing links like rvts, dwgs, images, ifcs and point clouds.
A model does build up redundant content throughout a project's lifetime. You should dive in deep and delete out some of this. This will include line styles, line weights, fill patterns, region fills, materials, sheets, text types, project parameters, shared parameters, families, annotations, tags, 2D Groups, 3D Groups, Keynotes
You will need to reposition your project coordinates to your new site along with your project north.Remove phases and design options.
Resolve all warnings.
Review view templates, filters and color schemes and delete any you don't need.
Change your Location to your new site address in the Sun Settings.Remove redundant worksets.
There's probably more but I can't think of them for now.
__________I'm the guy they come to when they didn't follow my recommended workflows.__________