You can use the REST API (Definitions - Update) to update the value of the release definition variable from a release task.

1. Go to the Agent Phase and select Allow Scripts to Access OAuth Token. See Use the OAuth token to access the REST API
2. Grant Project Collection Build Service (xxx) account the edit release pipeline permission. (Select the release pipeline --> ... --> Security --> Edit release definition set to Allow)
3. Add a PowerShell task in your release pipeline

4. Run inline script: (Update the value of variable v1030 in below sample)

   $url = "$($env:SYSTEM_TEAMFOUNDATIONSERVERURI)$env:SYSTEM_TEAMPROJECTID/_apis/Release/definitions/$($env:RELEASE_DEFINITIONID)?api-version=5.0-preview.3"
   Write-Host "URL: $url"
   $pipeline = Invoke-RestMethod -Uri $url -Headers @{
       Authorization = "Bearer $env:SYSTEM_ACCESSTOKEN"
   }
   Write-Host "Pipeline = $($pipeline | ConvertTo-Json -Depth 100)"
   
   # Update an existing variable named v1030 to its new value 1035
   $pipeline.variables.v1030.value = "1035"
   
   ####****************** update the modified object **************************
   $json = @($pipeline) | ConvertTo-Json -Depth 99
   
   
   $updatedef = Invoke-RestMethod -Uri $url -Method Put -Body $json -ContentType "application/json" -Headers @{Authorization = "Bearer $env:SYSTEM_ACCESSTOKEN"}
   
   write-host "==========================================================" 
   Write-host "The value of Varialbe 'v1030' is updated to" $updatedef.variables.v1030.value
   write-host "=========================================================="