You don't use LINQ to perform modifications to your data collection. If you want to modify the collection, use a normal foreach loop
foreach(var pr in ProductReviews)
pr.AvatarUrl = pr.AvatarUrl.Replace("URL", serverImageUrl);
Remember: "LINQ is for querying, not modifying"
If you want to replace URL using LINQ you do it as part of a query that returns a enumeration of new objects that represent your replacements:
ProductReviews.Select(pr => new { Review = pr, ReplacedUrl = pr.AvatarUrl.Replace("URL", serverImageUrl) } );
This will give you an enumerable of new objects that has the original and the replaced url. It's easier to include the original, assuming you need all the fields, as a single field but you could also list out the things you want:
ProductReviews.Select(pr => new {
pr.Id,
pr.CustomerId,
AvatarUrl = pr.AvatarUrl.Replace("URL", serverImageUrl)
} );
Or you can use an automapper to map your db objects to your client side objects and save a bit of this laborious typing
serverImageUrlvariable?