HOW TO Install this skin :

simply put the pist_glock19.vtf in your pist_glock18 folder. one for v_models and one for w_models

After you done that open your pak01_dir which will be found at C:\Program Files (x86)\Steam\SteamApps\common\Counter-Strike Global Offensive\csgo

Open it whit Notepad++
And change the pist_glock18 whit pist_glock19
On the basetexture line. 