The powershell I used is:
$site = Get-SPSite $portalUrl $taxSession = Get-SPTaxonomySession -Site $site $termStore = $taxSession.TermStores[$managedMetadataService] $termStoreGroup = $termStore.Groups[$termStoreGroup] $termSet = $termStoreGroup.TermSets[$termSet] $term = $termSet.Terms.Item($term) $term.SetLocalCustomProperty("_Sys_Nav_TargetUrl", $newUrl) $termStore.CommitAll()