The code is simple:
$farm = Get-SPFarm
$cacheService = $farm.Services | where {$_.Name -eq "AppFabricCachingService"}
Write-Host $cacheService
$accnt = Get-SPManagedAccount -Identity "DOMAIN\ACCOUNTNAME"
Write-Host "Managed Account" $accnt
$cacheService.ProcessIdentity.CurrentIdentityType = "SpecificUser"
$cacheService.ProcessIdentity.ManagedAccount = $accnt
Write-Host "Now Updating"
$cacheService.ProcessIdentity.Update()
Write-Host "Finished."
Only change "DOMAIN\ACCOUNTNAME". You will not need to use .Deploy() at the end, despite Microsoft using it.