First: Is it too *fast*, or does it *accelerate too hard*?
The former you would need to change the speed limits for the assist levels.
The latter you would need to change the current (torque) or power limits for the assist levesl.
You probably want to check the manual first for how the assist levels operate for this specific controller.
Few of these controllers even do any form of power control via PAS--they almost always only control speed via the assist levels, so that each level simply has a maximum speed, and pedalling at all engages PAS to the full speed of the presently-chosen assist level. The throttle then overrides that to vary the speed between zero and full.
If yours is one of the few that provides "torque simulation" or current / power control via the assist levels, then it will have a maximum current (or watts) for each assist level.
So if yours is like the latter, *and* your controller has customizable assist levels (which you can check it's manual to find out), *then* you can change what power each one uses.
But if it's like the former, *and* it has customizable assist levels, then you can change what speed each one uses.