Q:

How to Increase/Update Variable Group value using Azure Devops Build Definition? 

Am working on Azure Devops CI&CD.  Here, my release name must be in Version number using tags.  Am getting this with the help of Variable Groups, by adding tags and value to it.  Here am getting the tags value as constant like a static for every release like 1.1,1.2,1.3 etc. 
Now am trying to increase/update my tag value dynamically for every new release triggered after completion of my Build Definition successfully which looks like 1.1,1.2,2.1,2.2,3.1,3.2 etc. It is possible with the help of statically by variable group, but manually we need to update it. 
Is it possible to Increase/Update the tags value in Variable Group with the Build Definition tasks or other process. If possible, please suggest me to "How to done this?"  

A:

You can overwrite/update the value of the variables by using the logging command to set the variables again in Azure Devops Build pipleline:
Write-Host "##vso[task.setvariable variable=testvar;]testvalue"

To increase the value dynamically, you need to use the token $(Rev:.r).  You can custom the variables based on the $(Build. BuildNumber) or $(Release. ReleaseName)as they will increase the value dynamically...
Just reference this thread to custom the variables:https://github.com/MicrosoftDocs/vsts-docs/issues/666#issuecomment-386769445

UPDATE:
If you just want to update the value of the variables which defined in a specific Variable Group, then you can call REST API in build pipeline to achieve that:
PUT https://{account}.visualstudio.com/{ProjectName or ID}/_apis/distributedtask/variablegroups/{Variable Group ID}?api-version=5.0-preview.1

Content-Type: application/json

Request Body:

{"id":2,"type":"Vsts","name":"VG0926","variables":{"TEST0926":{"isSecret":false,"value":"0930"}}}

UPDATE2:
You can write a PowerShell script to call the REST API, then add a PowerShell task to run the script in your build pipeline: (Use the OAuth token to access the REST API)
Below sample for your reference:
$url = "$($env:SYSTEM_TEAMFOUNDATIONCOLLECTIONURI)$env:SYSTEM_TEAMPROJECTID/_apis/distributedtask/variablegroups/{Variable Group ID}?api-version=5.0-preview.1"
Write-Host $url

function CreateJsonBody
{

    $value = @"

{"id":2,"type":"Vsts","name":"VG0926","variables":{"TEST0926":{"isSecret":false,"value":"0930"}}}

"@

 return $value
}

$json = CreateJsonBody

$pipeline = Invoke-RestMethod -Uri $url -Method Put -Body $json -ContentType "application/json" -Headers @{
    Authorization = "Bearer $env:SYSTEM_ACCESSTOKEN"
}

Write-Host "New Variable Value:" $pipeline.variables.TEST0926.value

UPDATE3:
Well, tested again, below scripts works for me as well.  You can try it, just replace the parameters accordingly:
# Base64-encodes the Personal Access Token (PAT) appropriately
$base64AuthInfo = [Convert]::ToBase64String([Text. Encoding]::ASCII.GetBytes(("{0}:{1}" -f "","PAT here")))
$url = "https://dev.azure.com/xxx/Test0924/_apis/distributedtask/variablegroups/1?api-version=5.0-preview.1"

$json = '{"id":1,"type":"Vsts","name":"VG0928","variables":{"TEST0928":{"isSecret":false,"value":"0931"}}}'
$pipeline = Invoke-RestMethod -Uri $url -Method Put -Body $json -ContentType "application/json" -Headers @{Authorization=("Basic {0}" -f $base64AuthInfo)}
Write-Host "New Variable Value:" $pipeline.variables.TEST0928.value 

