Is there a way to change the replication interval to more than 1 day between the main and back-up servers for BigFix?
We need this for the upgrade of server component to 9.5.
Assuming you are referring to DSA, then certainly! Do note that replication is bi-directional by default, and so, you may wish to change the interval for more than 1 replication link:
Windows Root Servers:
https://www.ibm.com/support/knowledgecenter/SSQL82_9.5.0/com.ibm.bigfix.doc/Platform/Config/c_change_the_replication_interva.html
Linux Root Servers:
https://www.ibm.com/support/knowledgecenter/SSQL82_9.5.0/com.ibm.bigfix.doc/Platform/Config/c_change_the_replication_interva_linux.html
Yes, that I knew but the max interval there is 1 day.
We need more than that, hence the question 
Ah, my apologies, I misunderstood the question.
There is not a way to configure the interval to be longer than 1 day, no. That said, disabling the Root Server and FillDB Services on the backup/secondary server will essentially disable its replication from the primary for the purposes of upgrade (note that this isn’t really a necessary step).