Kleopatra "change owner trust" to my certific

Newbie question, please point me to the correct place to ask if this is not it.

I am unable to specify that an imported certificate is in fact MY certificate (the option shows in the drop-down, but it is shaded out). Because I’ve upgraded to a new computer, I installed Gpg4win and hoped to easily decrypt files that I had encrypted previously (using pgp). I am able to find my existing certificate and import it, but I can’t get Kleopatra to recognize it to be MINE (and thus usable, presumably, to decrypt file (after prompting for the pass-code, I hope) that I encrypted previously on my prior comptuer/setup using pgp.

What am I missing? Thanks for any hand-holding