Hello, hope you are doing well!
No, once submitted the CAT application form can't be edited. So it is very important to fill the form without any error in the first and last attempt. Information that cannot be edited in CAT form include personal details, academic details and selection of test cities.
Replying to your query from the comments' section-
Application process is the foremost step and its not that big a problem if you have done a minor mistake. If there's too much difference in the marks you have got and the marks you have entered then its definitely a problem. Because what happens is if you have mentioned more than the marks you have got then at the time of document verification you might be doubted to have intentionally entered more marks to get selected. And if you have entered too less marks than that of what you have actually got, you might not be considered eligible only. If your mistake is not causing these two future events to happen then its fine. Your mistake will be rectified when you go for your document verification i.e. the final step to seek admission. You may have to mention you made a mistake or they will find the mistake themselves while tallying your marksheets with application details and they will give you a chance to rectify.
So, don't worry if you have not made an extreme mistake in your application which might create extreme situations in the future.