iam not a fan of ISSU, it has some drawbacks i have seen before.
my traditionla way approach below :
1) Save configuration before starting.
2) Check there is enough space on devices
3) Copy over firmware to the bootflash, both kickstart firmware and system files.
copy to disk copy kickstart.bin bootflash:
copy to disk copy orginal.bin bootflash:
install all kickstart kickstart.bin system orginal.bin
4) Once the secondary responds to pings again, logon and check that the secondary nexus has come back up with the right firmware
Checking that vPC (protocol used for the network resilience), is up correctly by doing a show vPC, before upgrading the primary.
5) Repeat process for Primary device
checks
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show vPC
show version