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If you mean by "upgrade" replace the switch's CPU, that's not done. Generally the whole switch would be upgraded (or on a chassis switch, perhaps a supervisor card).
If's very unusual to see a L2 or L3 switch with a very high CPU except when there's some "issue". The reason being, L2 and L3 switch off-load almost all data plane forwarding to dedicated ASICs, the CPU normally only deals with control plane needs.
On the 3560/3750 series, if TCAM resources are exceeded, some ASIC processing is then done by the CPU, which can overtax the latter.
What you might first do is search the main Cisco web site for troubleshooting high CPU utilization on the 3560/3750 series switches.
