Do you want to be able to edit the content?
Ok do you want a quick scale?
Make a new A4 sized document
And it will import the indesign file into the A4 page and scale everything up in size.
You can then edit the original A5 size, then save, and open the A4 document and update all the links.
But if you want to make the A5 document the A4 size - there is no quick way but there is a bit of shortcut, hoping you have used margins and guides to position things on the pages.
If you go to the A5 document - make a Save As (for safe keeping of the oringinal)
Then Go to Layout>Layout Adjustment and Enable it
Then go to File>Document Setup and change the page size to A4
You will have to go to page to page to position things.
A quick way to do this is to open the Original A5 (aren't you glad you kept a copy!)
Then Tile the windows so that you can see the A4 and A5 beside each other.
You can then go page by page and position things as necessary.
Hopefully you have used styles and can easily change the point size, leading etc. to help fill the pages.
Keep things consistent and best of luck