First, open the TortoiseGit Settings dialog. On the left, select Diff Viewer.
Under Configure the program used for comparing different revisions of files, select External and type the following into the box.
WinMergeU /r /e /x /u /wl %base %mine
Next, select Merge Tool (on the left). Again, select External and type the following.
WinMergeU /e /u %merged
Now, wherever TortoiseMerge use to show up, you should see WinMerge instead.