Syntax
anUnescapedString = aString.unescapeURI(codepage)
anUnescapedString = unescapeURI(LITERAL, codepage)
Semantics

The string encoded as defined in RFC 2396  is unescaped to a UTF-8 string. The codepage defines the source encoding, e.g. URL parameters in links are in the same encoding  as the website (e.g. ISO-8859-1). The return value is a string.

Parameters of URLs must be decoded value by value.

SubstitutablesaString Can be any variable or object attribute having the type String.  
codepage A string with the name of the target codepage.
LITERAL String literal.
Examples
set unescaped = unescapeURI("M%fcller", "iso-8859-1"));
The string unescaped will have the value Müller.

Example File (Builder project Basic Modeling/Data):

<your example path>\Basic Modeling\Data\uml\urlEncoding.xml