How to remove top window joist?

[Toastandjam]

I have a rotten joist (7cmx 7cm) right next to the window that I've been told is only holding up small stone, the lintel is holding up the rest of the wall.
I need to remove it which is the easy part, but how do I put in a new one? Is it a case of creating enough space on the left to slot it in and then slide it along to the other side to make it sit there?

 

[Koolpc]

You need to make it all safe first before you start demolishing it...

 

[Toastandjam]

You need to make it all safe first before you start demolishing it...

I thought about an acrow prop, but then how do I get to it? The joist infront is doing most of the heavy lifting....