Budgies often don’t begin to incubate eggs until the first 2 or three have been laid. You can just throw it away, it won’t have started developing yet.
Have you actually seen the pair mating? The egg may not even be fertilised as a female can lay eggs with or without a male present.
To move her out of this behaviour, you need to make the following changes:
Have a major reshuffle of the cage contents (Things are changing - not a good time to lay eggs)
Reduce her daylight hours to a maximum of 8 (Winter - not a good time to lay eggs)
Reduce/stop high protein/fat foods ie no egg and limit seed to 1.5-2 tsp per bird per day (Not an excess of foods - not a good time to lay eggs)
Consider temporarily separating your pair - if you’ve noticed mating I would certainly do this (can be same room adjacent cages).
Remove any potential nesting material and nest sites - block off access to the drawer and view all their accessible area through a hormonal budgie’s eyes, looking for dark enclosed nooks and crannies that she may find appealing.
Personally, I would confine her to her cage for about a week, then if she hasn’t laid any more, observe closely as you reassess her behaviour outside of the cage.
Once ‘back to normal’, you can put them back together and take note of the advice in this link:
Breeding and allowing birds to incubate eggs ideally needs prior work to have most chance of a healthy outcome. The birds should be on a conditioning diet beforehand and the owner needs to have the knowledge and equipment available to be prepared for intervention when things go wrong.
Best of luck