Accidentally I've set the PMDG Unpainted texture as default. Now I want to revert it and restore the original PMDG texture. How should I proceed?

It's highly recommended to not set any advanced add-on aircraft as the "Default" for free-flight when FSX is first opened, if that's what you mean by "default".  It can lead to many undesired issues (not to mention a slower FSX startup time).

 

Regardless though, to select other liveries, just be sure the "Show all variations" check-box at the bottom of the "Select Aircraft" browser is checked and you should see all liveries.

It's highly recommended to not set any advanced add-on aircraft as the "Default" for free-flight when FSX is first opened, if that's what you mean by "default".  It can lead to many undesired issues (not to mention a slower FSX startup time).

 

Regardless though, to select other liveries, just be sure the "Show all variations" check-box at the bottom of the "Select Aircraft" browser is checked and you should see all liveries.

This is not the case. I have downloaded via Ops Center the Unpainted livery for the T7. Accidentaly, I have setted the Unpainted livery as the default, replacing the original PMDG texture. Now, I want to revert this situation. Is it possible?

I am not sure how one "sets a livery as default", so let me guess:

You downloaded the Unpainted livery, then either did one of the two:

 

(1) Modified the aircraft.cfg file so that the unpainted livery is under the [fltsim.0] heading

 

or

 

(2) Renamed the Texture.PMDG_u folder to Texture.pmdg

 

or

 

(3) Copied the contents of the Texture.PMDG_u folder unto the Texture.pmdg folder

 

Do any of those describe how you "replaced the original PMDG texture"?

Negative. I've downloaded the Unpainted livery and when I tried to delete, the Ops Center reported that it was not possible to delete the House Texture. So, the Ops Center has made it as default. After that, accidentaly, I had deleted the original PMDG House texture at the Ops Center. Now I want her back.