How to Update List Items in SharePoint using PowerShell Script?
Well, the basic syntax for updating a list item goes as:
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#Get Web and List 
$Web = Get-SPWeb "http://sharepointsite.com"
$List = $Web.Lists["ListName"]
 
#Get the List item to update
$ListItem = $List.GetItembyID($ItemID)
  
#Set Column values
$ListItem["ColumnName"] = "New Value for the Field!"
  
#Update the Item, so that it gets saved to the list
$ListItem.Update()

Update SharePoint list item using PowerShell script
Lets take an another example:
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Add-PSSnapin Microsoft.SharePoint.PowerShell -EA SilentlyContinue
 
$web = Get-SPWeb http://SharePointSite.com
 
#Get the Contacts List
$List = $web.Lists["Contacts"]
 
#Get the List Item by its Title field value
$ListItem = $List.Items | Where {$_["Title] -eq "Salaudeen Rajack"}
 
#Set "Department" column value
$ListItem["Department"] = "IT"
 
$ListItem.Update()

Update All List items using PowerShell script:
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$web = Get-SPWeb http://SharePointSite.com
 
#Get the Contacts List
$List = $web.Lists["Contacts"]
 
#Get All Items by update its title
$ListItems = $List.items
  
#Iterate through each item
foreach($Item in $ListItems)
{
    #Update the value of the "Title" column
    $item["Title"] = "Title Updated!"
  
    #Update the item
    $item.Update()
}

How about Updating Selective List Items in Bulk:
Lets update list item by its id field.
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Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue
 
#Variables
$SiteURL = "https://portal.crescent.com/pipeline/"
$ListName = "Deals"
 
#Get the List
$List = (Get-SPWeb $SiteURL).Lists.TryGetList($ListName)
 
#List of Item IDs to update
$ItemIDs = 16,52,69,70,97,170,194,195,196,220,259
 
If($list)
{
    foreach($ItemID in $ItemIDs)
    {
        #Get Item by ID
        $ListItem = $List.GetItembyID($ItemID) 
 
        #Update List Item Fields
        $ListItem["Health and SafetyRisk"]="Low"
        $ListItem["Environmental Risk"]="Medium"
        $ListItem["Social Risk"]="High"      
 
        $ListItem.SystemUpdate()
        Write-host "Updated Item ID:"$ItemID
    }
}


#Read more: http://www.sharepointdiary.com/2015/03/powershell-script-to-update-sharepoint-list-item.html#ixzz5hEe46w9A