Using SetTexture to change the texture of a frame created by another addon?

[Krainz]

GwGarrisonButton is a frame created by GW2 UI

The goal with that code is to change gwbutton's texture to another one.

If I get it to work I'll start employing the same to other frames

UIMod_UpdateInterval = 0.3; -- How often the OnUpdate code will run (in seconds)
 
 
f.TimeSinceLastUpdate = 0
 
f:SetScript("OnUpdate",function(self,elapsed)
        local gwbutton = GwGarrisonButton
        local garrisonType = C_Garrison.GetLandingPageGarrisonType()
        local covenantID = C_Covenants.GetActiveCovenantID()
    self.TimeSinceLastUpdate = self.TimeSinceLastUpdate + elapsed;
 
 
    if (self.TimeSinceLastUpdate > UIMod_UpdateInterval) then
        print("UIMOD OnUpdate");
                if gwbutton and gwbutton:IsShown() and gwbutton:IsVisible() and (covenantID ~= 0 or garrisonType == 111 or garrisonType == 3) then
                        print("UIMOD OnUpdate: GW2 UI Garrison Button is shown. Editing texture.");
                        gwbutton:SetTexture("Interface/AddOns/UIMod/textures/icon-1")
                end
 
 
        self.TimeSinceLastUpdate = 0;
        UIMod_UpdateInterval = 30.0;
    end
end)

However this returns me an error. Apparently I can't use SetTexture that way. I tried googling around but only found people talking about changing Blizzard's default textures.

How do I change the texture of a frame created by another addon?

Maybe a XML file could help in this case?