Hello,
You're asking a good question and it's best to go about addressing your issue the right way. While there's not much you can do to change the fact that you have some low undergrad grades, you can be proactive NOW and take steps to address this weakness, which will also show you are proactive. A few things to consider - is there a pattern in terms of which classes you had lower grades in? If, for example, you had lower grades in quantitative classes, you want to focus on this area and try to take a few quant classes soon and really focus on these to show you can do well in this area.
Admissions will like to see you take these classes for a grade (since you're hoping to have a much better grade, now that you're completely focused on these) and you can do this through a number of Continuing Education programs. If you are near the NYC area, NYU's School of Continuing and Professional Studies offers a wide range of pre-MBA or MBA-focused classes (such as Calculus, Economics, Statistics) and you can take these for a grade.
URL: https://www.scps.nyu.edu/content/scps.html
Some reputable online programs are also available, such as UC Berkeley Extension School:
https://extension.berkeley.edu/publicVie ... ethod=load
If you are focusing on applying to a particular MBA program, you can reach out to them and ask which Continuing Ed programs they recommend since there might be other programs that they know are reputable. In terms of which classes to take, it's probably suggested that you focus on analytical classes and some of the most popular pre-MBA classes are Calculus, Economics, Statistics, and Accounting.
Good luck and hope this helps! Feel free to reach out if you have more specific questions.
Thanks,
Dina