Based on the answer to this other question, I created a test script to modify the link of a FunctionResource. The following code snippet assumes that the script receives a resource's GUI, a new primary key, and a table entry parameter id.

The script is referencing our SLSRMLibrary for easy access to the ResourceManager:

var resourceManager = SrmManagers.ResourceManager;

// Get the FunctionResource, main element and function element
var resource = (FunctionResource)resourceManager.GetResource(
Guid.Parse(engine.GetScriptParamValue<string>("FunctionResourceId"))) ??
throw new Exception($"Resource {engine.GetScriptParamValue<string>("FunctionResourceId")} not found");

// List the current entry points
foreach (Tuple<int, string> entry in resource.LinkerTableEntries)
{
engine.GenerateInformation($"DEBUG:({entry.Item1},{entry.Item2})");
}

// Setting a new PK. No extra validation, so the new values are assumed to be valid
var newPK = engine.GetScriptParamValue<string>("NewPK");
var newTableId = engine.GetScriptParamValue<int>("TableId");
resource.PK = newPK;
resource.LinkerTableEntries = new Tuple<int, string>[]
{
new Tuple<int, string>(newTableId, newPK),
};
resourceManager.AddOrUpdateResources(new Resource[] { resource });