If you have already sent an invitation to a candidate but they are unable to complete it in the allocated time it is possible to extend it without sending another test invite as long as the test is still valid.
Pick the candidate and the necessary invite. Then click on the 'Extend' button to prolong the validity of the invite for the necessary amount of days. Check how to do this on the Gif below.
If you need to extend the default validity period of the invitation, you can do that here: Profile >> Account settings >> Validity period. Please note that only a user with certain permissions can do it.