Update release pipeline variable.ps1

$url = "$($env:SYSTEM_TEAMFOUNDATIONCOLLECTIONURI)$env:SYSTEM_TEAMPROJECTID/_apis/Release/definitions/$($env:RELEASE_DEFINITIONID)?api-version=5.0"

#Get definition
$pipeline = Invoke-RestMethod -Uri $url -Headers @{
    Authorization = "Bearer $env:SYSTEM_ACCESSTOKEN"
}
 
#Update variable 
$pipeline.variables.MyVariableName.value = "New value"

#Update the modified object to JSON
$json = @($pipeline) | ConvertTo-Json -Depth 99

#Update the definition via api
$updatedef = Invoke-RestMethod -Uri $url -Method Put -Body $json -ContentType "application/json" -Headers @{Authorization = "Bearer $env:SYSTEM_ACCESSTOKEN"}