1) Edit client tasks for that server. Break inheritance on the scan task, modify the time for that task on that one server, and save it.
2) Create a subgroup under the existing group. Modify the scan task's time for that group, move the server into that subgroup.
#2 is a little cleaner (it's easy to forget that you modified the time for that one server), but either works.
I duplicated the existing client task and assigned the new task to the server alone .so that i can know which servers have which task assigned .
Its also helps me in audit purpose.