You can either click the split topic button above your quick mod tools, or you can select 'split this topic' from the drop down menu. When splitting a topic you have two options, either splitting selected posts, or splitting from selected posts.

For splitting selected posts, you need to make a new topic title and select the forum you want to move the split posts too. You can then check the boxes to the right of each post of the topic you are splitting. Finally click 'split selected posts'. It will show you the new title and the forum you have chosen for the new topic and ask you to confirm.

For splitting from selected posts you need to make a new topic title and select the forum you want to move the unselected posts too. You can then check the boxes of all the posts you want to KEEP as part of the topic you are splitting, and leave the posts you want to make a new topic with unchecked. Click 'split from selected posts' button and it will show you the new title and the forum you have chosen for the new topic and ask you to confirm.