public/release/Update-PSTSReleaseDefinition.ps1

function Update-PSTSReleaseDefinition
{
    param(
        [Parameter(Mandatory = $true, ValueFromPipelineByPropertyName = $true)][string] $project,
        [Parameter(Mandatory = $true, ValueFromPipelineByPropertyName = $true)][object] $releaseDefinition
    )

    process {

            _PSTSAPI `
                    -subDomain "vsrm" `
                    -project $project `
                    -resource "Release/Definitions" `
                    -method "PUT" `
                    -contentType "application/json" `
                    -body (ConvertTo-Json $releaseDefinition -Depth 100)`
                    -apiVersion "api-version=5.1"
            }

}