How do I change a file's attribute using Powershell?

You can use the good old dos attrib command like this:

attrib -a *.*

Or to do it using Powershell you can do something like this:

$a = get-item myfile.txt
$a.attributes = 'Normal'

As the Attributes is basically a bitmask field, you need to be sure clear the archive field while leaving the rest alone:

PS C:\> $f = get-item C:\Archives.pst
PS C:\> $f.Attributes
Archive, NotContentIndexed
PS C:\> $f.Attributes = $f.Attributes -band (-bnot [System.IO.FileAttributes]::Archive)
PS C:\> $f.Attributes
NotContentIndexed
PS H:\>

From here:

function Get-FileAttribute{
    param($file,$attribute)
    $val = [System.IO.FileAttributes]$attribute;
    if((gci $file -force).Attributes -band $val -eq $val){$true;} else { $false; }
} 


function Set-FileAttribute{
    param($file,$attribute)
    $file =(gci $file -force);
    $file.Attributes = $file.Attributes -bor ([System.IO.FileAttributes]$attribute).value__;
    if($?){$true;} else {$false;}
}