This script runs through a list of items in a SharePoint list and changes them to upper case and then updates the list item.
$webUrl = "https://xxxxx"
$listName = "Fleet Contacts"
# Open web and library
$web = Get-SPWeb $webUrl
$list = $web.Lists[$listName]
$items = $list.items
foreach ($item in $items)
{
$Item["Command_x0020_Name"] = $Item["Command_x0020_Name"].ToString().ToUpper()
$Item["Command_x0020_Name"]
$Item.Update()
}