Function not completing updating variables in the Script Unity C#

I suggest you spend some time reading about variable scoping in C#. You can think of these as blocks enclosed by { }.

When you declare a variable inside of a scope (such as with float XPlevel = XPImport), that variable only exists inside of that scope:

public void statsImport(float XPImport)
{
    // start new method scope

    // declare variable `XPlevel` inside of that scope
    float XPlevel = XPImport; 
}

void Update()
{
    // start new method scope

    // `XPlevel` doesn't exist here
    Debug.Log(XPlevel);
}

Instead, what you probably want is something like this, where you make XPlevel a field inside of the class-scope:

public class MyComponent : MonoBehaviour
{
    // declare class-level field
    private float XPlevel;

    public void statsImport(float XPImport)
    {
        // `XPlevel` exists here
        XPlevel = XPImport; 
    }

    private void Update()
    {
        // `XPlevel` exists here
        Debug.Log(XPlevel);
    }
}

If you want to be able to edit the value of XPlevel inside of the Editor, add the [SerializeField] tag. This has the same effect as making XPlevel public, but only allowing the field to be set inside of your script:

[SerializeField]
private float XPlevel;

Once you're comfortable with that, you could also make XPlevel a property instead:

[field: SerializeField]
public float XPlevel { get; private set; }