Below is an example script to find and replace all instances in all fields below the root item using Sitecore Powershell Extensions.

Get Fields using Standard Values

$rootItem = "master:/sitecore/content/Home"
$needle = "vegetables"
$newNeedle = "pizza"
$count = 0
Get-ChildItem $rootItem -Recurse | ForEach-Object {
    $currentItem = $_
    Get-ItemField -Item $currentItem -ReturnType Field -Name "*" `
    | ForEach-Object{ 
        if($_.Value.Contains($needle)){
            #output itemid and field name
            Write-Host ("[" + $currentItem.ID + ": " + $_.Name + "]")
            $newHaystack = $_.Value.Replace($needle, $newNeedle);
            $currentItem.Editing.BeginEdit()
            $currentItem[$_.Name] = $newHaystack
            $currentItem.Editing.EndEdit()
            $count++
        }
    }
}

Write-Host "Updated " $count " Found Values"

The above script will return something pretty simple like:

[{A351623A-003E-486B-A35B-EDD69E335451}: Description]
True
[{A351623A-003E-486B-A35B-EDD69E335451}: Page Title]
True
[{586C7A62-FE78-4C00-9816-0DF8E3FE5849}: Html]
True
[{5A9BC349-5BDB-4D14-8FB4-D4F6EBB593B1}: Html]
True
Updated  137  Found Values

There may be many other ways to do this same task. Heck, probably several of them more efficient than this (and maybe less agressive as well) but I thought I’d take a look-see how it could be done. There ya go.

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