KB: Edit primitives locked in schematic template such as of title block

Solution Details

A schematic sheet template is a regular schematic sheet, but saved with the *.SchDot extension.  Templates can be referenced from a local location - in which case the template is stored as a *.SchDot file, or from a managed location - these are stored on a managed content server such as Concord Pro or Altium 365.
The template (*.schdot) referenced by a given schematic sheet (*.schdoc) can be confirmed in Properties panel, on General tab, under Page Options section.
https://www.altium.com/documentation/altium-designer/setting-up-schematic-document#!page_options_sch:~:text=Formatting%20and%20Size-,Template,-mode%20%E2%80%93%20set%20the

In general, the primitives can be edited only within the very original *.schdot in which they are instantiated the first time, while parameter values indirectly denoted by Special Strings is referred from the current document/project:
https://www.altium.com/documentation/altium-designer/creating-schematic-templates#:~:text=the%C2%A0active%20document.-,Note,-that%20text%20and

It is conceivable to have a schematic template (*.schdot) referencing another template (*.schdot), or so-called nested template or template-in-a-template.  If you have some primitives in a sheet that you cannot edit, it is most likely because it belongs to another sheet template, where you would need to edit them.  You can locate the template used for the current sheet in Properties panel, on General tab, under Page Options section .

You may find the following video to be useful. It is recorded on AD17 but the principle has remained the same across all versions, and the behavior is as intended so as to mitigate accidental tampering.
https://www.youtube.com/watch?v=pbi4VyNv7k8

As an alternative approach, in case you've misplaced the original *.schdot from which you've derived the current *.schdot you are trying to edit now, you can try making use of scripting, described in the forum thread below:
https://forum.live.altium.com/#posts/218073/628132

For any image object as a part of *.schdot, such as a company logo, it is also worth noting that the option 'Embedded' allows a copy of the image to reside inside *.schdot, so as to make the template portable:
https://www.altium.com/documentation/altium-designer/schematic-drawing-objects#image