@Katherine59,
I understand that you have concerns regarding the certificates when using any site.
If you are getting a certificate error on every website you visit, then it is either an issue with the browser or there could be malware on the system. It isn't very likely that the certificate of every site you visit has a certificate problem.
To answer your question, the certificate isn't something that is user manageable or can be changed by the user. This is a certificate that is issued to the site by a CA to verify their identity with. The content of the certificate being use is then verified and a certificate warning is then issued if something isn't right (also depends on what isn't right). Some minor issues don't prompt the certificate warning but put a little warning sign next to the link in the address bar. For example, Google Chrome puts a yellow triangle on the lock icon next to the address for a minor issue.
Boot into safe mode with networking to verify if the issue exists there as well. This can be done by turning the system on and pressing the F8 key about once a second. Once you are in safe mode, try accessing a site that produced the certificate error.