If you look into the error message above which you have posted : "KB2600640 is not installed on this computer or the update is incompatible with the current installation of KB2600640" . KB KB2600640 s nothing but UR6 for CRM 2011. Now if you can check the Requirements for installation of UR13 it shows
Prerequisites:
•This update must be installed on a computer that is running Microsoft Dynamics CRM 2011 Update Rollup 6 or a later Microsoft Dynamics CRM 2011 update rollup.
Refer: www.microsoft.com/.../details.aspx
If you go to the www.microsoft.com/.../details.aspx which is UR 6 for CRM 2011
Prerequisites:
•This update must be installed on a computer that is running Microsoft Dynamics CRM 2011.
So, if you look into the client machine where you are trying to install UR13 and it's failing with the above error message you can not see Ur6 for CRM 2011 installed into the Programs and features list that mean you can not installed any other Update rollup successfully before applying the base UR6, So, first install UR6 and then you may skip all other and directly install UR13 if you wish as the UR13 will be having all the cumulative updates of UR7+UR8+UR9+UR10+UR11V2+UR12.
Hope that answers the thread here.
Thanks,
Mohammad