Run the installation files for the 757 and 767 Base Packs, select REPAIR when it asks. Repair or uninstall are the choices.
Any changes you have made to any of the original files, will be reverted back to their original form when you do the repair.
And if you have added extra liveries, they will need to be redone afterwards as well.
I suggest you back up any files that have been changed, BEFORE you do the repair.

Mark