Navigate to Data⇒Organisation⇒Customer

Locate the customer you wish to amend, using the Search function if required. Double click or click the edit icon to open the customer record.

On the basic tab you will see Ranking with the numbers 1-5 after it, these a correspond to the ranks 1-5, where 1 is the highest priority and receives the fastest service (see also buffer time, and SLA)

Slecet the service level required and then press Save