How do I replace a line in a file using PowerShell?

Try the following:

$file="D:\home\App_Config\Sitecore.config"
$regex = '(?<=<setting name="Media\.MediaLinkServerUrl" value=")[^"]*'
(Get-Content $file) -replace $regex, 'https://newurl.com' | Set-Content $file

^{* Re Set-Content: In Windows PowerShell it uses your system’s legacy single-byte character encoding by default (based on the active ANSI code page), so you may want to use -Encoding to control the output file’s encoding explicitly; PowerShell [Core] 6+ defaults to BOM-less UTF-8.}

• Also note the required (...) around the Get-Content call to ensure that the pipeline can write back to the same file that Get-Content has read from.
• If there’s a chance that the opening tag in question (<setting ...>) spans multiple lines, use
  Get-Content -Raw $file (PSv3+) to read the entire file content as a single string (thanks, deadlydog);
  without -Raw, Get-Content returns an array of strings, representing the input lines.

Due to using a regular expression to match your existing setting, any text currently inside value="..." is matched, so this command will work even when run repeatedly.

By contrast, what you tried uses an effective literal (... value=" ") to find what to replace, and after the 1st – successful – run, that literal no longer matches, and subsequent runs have no effect.

The command above uses a streamlined approach to replacement:

• (?<=<setting name="Media.MediaLinkServerUrl" value=") is a look-behind assertion ((?<=...)) that matches, but doesn’t capture what it matches: it finds the part up to and including the opening " of the value you’re trying to replaces, without making that prefix a part of what will get replaced.

• [^"]* then matches the entire value, up to, but not including the closing ". ([^"] is a character set that matches any character other than (^) a ", and * finds any (possibly empty) sequence of such characters.

• Therefore, because the regex captured only the value itself, all you need to specify as the replacement string is the new value.