What can I do to adjust my invisalign retainer after a crown?

You can't . If the crown is smaller than the tooth it is replacing you won't need any adjustment. If the retainer does not fit well over the crown you will need to get a new one.
Need a new retainer. In most cases, dentists are able to match the original anatomy of a tooth when fabricating a crown so that the invisalign retainer fits. However, this is not possible in all cases. The invisalign retainer cannot be manipulated to fit the new crown, as attempts to do so will distort it, with negative consequences. Consult your dentist/orthodontist to have a new retainer fabricated.
You Can't. More than likely you can't. See your dentist.
New one . You will. Of be able to adjust and invisalign retainer. The fit is critical, to keeping your teeth in the proper position so a new retainer will need to be made.
New trays. Have your orthodontist take new impressions and order new trays for you.
Sometimes. The retainer may or may not be able to be adjusted once the crown is fabricated. Be sure to bring this to the attention of the dentist who is making the crown. He\she will be able to advise you whether or not a new retainer will be necessary.
Need new one. As the finish of a clear aligner case is most often not as finely tuned as a braces case, it's harder to maintain post-treatment stability for aligner cases. I've not been pleased with the results when trying to retro-fit a clear tray retainer. Better to take a new impression and make a new retainer.