Answer:
In both cases, rejecting and accepting is correct to send an email or make a call.
If you accept, you can also go in person and accept the terms of the job.
Step-by-step explanation:
Accept the job:
There are many ways to accept a job, many times someone accept a job because they are interested in the terms they propose such as the salary or the schedule they give you to work.
The best way to accept it is in person, clearly speaking what you expect from the company and thanking for the opportunity to work with them.
Reject the job:
Many people reject a job because of economic instability, bad company references, heavy schedules or a counteroffer by another company.
The best way to reject the job by sending an Email in which you give a reason why you do not accept without going into details, without looking for complicated phrases or a style too far fetched and formal.