Driving a 6v 250ma motor by using 12v 2A adapter
I would recommend to use a buck or step down converter. You can find cheap buck converters on ebay or aliexpress. You need to find a buck converter that works at 12Vin and has a fixed output voltage of 5V (or adjustable). For example, something like shown below would do the job.
The advantage of using a buck converter compared to a linear regulator is that it won't get as hot. If you use a linear regulator for the job, you will also need a heatsink that can handle $$Ploss = (12-5)V*2A = 14W$$
A 14W heatsink will be massive. With a buck converter you won't need a heatsink.