看换岗，如果坐地铁的话，可以坐到green park,然后穿过格林公园，就到白金汉宫了。 How to say？

不知道你的“观看换岗”是什么意思？
后面的几句，可以这样说：
if you take the underground/ go by underground , you can get off at the Green Park stop, after walking through the Green Park, you will see Buckingham Palace

If you want to see the guard change,you can take underground and get off at Green Park Station. Then you will see Buckingham Palace.

我一星期前刚从伦敦回来，白金汉宫好像是在St James's Park下，走过去十分钟。 友情提醒下，现在白金汉宫的guard change是每两天一次的。