How to edit title of a pull request which is already merged in Azure repos?

I have a pull request, which is already marked as "Completed" but I would like to edit the title of that pull request. I cannot find the option to edit it.

I have already searched on different platforms if this option is disabled by any settings but still no luck.

Solution

It is not possible to update title of PR that have been completed. You can neither from the web UI nor use Azure DevOps API/CLI to update the title of completed PR.

I tried to call the API "Pull Requests - Update" to update the title of a completed PR.

# update-PR.ps1

$organization = "organization name"
$project = "project name"
$repositoryName = "repository name"
$pullRequestId = 71
$newTtitle = "new title of PR"
$pat = "Personal Access Token"

$uri = "https://dev.azure.com/${organization}/${project}/_apis/git/repositories/${repositoryName}/pullrequests/${pullRequestId}?api-version=7.0"

$base64AuthToken = [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes(("{0}:{1}" -f "", $pat)))
$headers = @{
    Authorization = "Basic $base64AuthToken"
    "Content-Type" = "application/json"
}

$body = @{title = $newTtitle} | ConvertTo-Json -Depth 5

$updatePR = Invoke-RestMethod -Method PATCH -Uri $uri -Body $body -Headers $headers
Write-Host ($updatePR | ConvertTo-Json -Depth 10)

It returns the following error message.

TF401181: The pull request cannot be edited due to its state.