Update default column values for libraries in different sites collections

Question

Hello I am working on a powershell script to update the default values for a specific column (Year) from 2016 to 2017. We have several project sites (each projectsite is a sitecollection). I would like to update all the column values for all the sites in the different site collections

I have created the following, but isn't working:

Add-PSSnapin Microsoft.Sharepoint.Powershell
$site = Get-SPSite "http://xxx.westeurope.cloudapp.azure.com/sites"
$sitelists = foreach ($web in $site.AllWebs) {
foreach($list in $web.lists){ $list }}
$list = $Web.Lists["Documents"]
$field = $list.Fields["Year"]
$field.DefaultValue = "2017"
$field.Update()

Answer 1

This works:

$w = Get-SPWeb "http://sharepoint"
$l = $w.lists["Shared Documents"]
$f = $l.Fields["Year"]
$f.DefaultValue = "2017"
$f.Update()

Or this:

if ((Get-PSSnapin -Name Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue) -eq $null)
{
    Add-PsSnapin Microsoft.SharePoint.PowerShell
}

$site = Get-SPSite "http://sharepoint"
foreach($web in $site.AllWebs) 
{
    foreach($list in $web.lists){
        $list = $Web.Lists["Documents"]
        $field = $list.Fields["Year"]
        $field.DefaultValue = "2017"
        $field.Update()
    }
}

One more example:

$allWebs = Get-SPWebApplication "http://yourWebAPP" | Get-SPSite -Limit All | Get-SPWeb -Limit All 

foreach($web in $allWebs)
{
     foreach($list in $web.lists){
        $list = $Web.Lists["Documents"]
        $field = $list.Fields["Year"]
        $field.DefaultValue = "2017"
        $field.Update()
    }
}

You will want to add error handling, checking if objects exists, and disposing of objects.