When I type in the right marriage it goes to the bottom of the marriage box. I'd have to delete all the other marriages to go to the top. Can the placement be reversed? I source my marriages too.
The reason it is probably going to the bottom of the list is that it has a later date than the others.
If sources have been attached that actually relate to banns or licence details, these will naturally have an earlier date than the marriage ceremony event. The event displayed (on the Person page) is always the earliest date from the sources attached - assuming the details have been carried across during the source linking process.
I suppose if the dates are all identical, the last one added might then go to the bottom of the list. However, whatever the reason, to get your preferred option (probably the correct one) to get to the top of the list, then, unfortunately deleting those entries above it is the only option.
The issue has been raised many times in the past, but the Marriage Event section appears to attract little interest when it comes to the developers deciding their Family Tree enhancement priorities. There are a number of other (marriage event related) issues that users have highlighted as in need of being addressed.