Q: Can you upgrade your matric marks?
A: You can in South Africa rewrite your matric subjects to either increase your respective marks or upgrade your pass level. You can do this at the high school where you had written your matric or through another institution. Even some high schools offer a post matric allowing students to redo courses in order to get an improved mark/grade.
Most private colleges offer this facility or courses which are matric equivalent.  When looking for a course which is a Matric equivalent, you are looking for something which will give you an NQ (National Qualification) Level 4 qualification.  Please make sure that the college you go to is a registered FET College (recently changed to TVET)* so that the qualification you receive is valid.  No-one wants to spend another year doing something only to find that it is not worth anything – what a waste of time and money and then you have to start again!

You can find listings of both National and Private FET/TVET* colleges on the South African Basic Education and Higher Education & Training websites.
Further if you do not qualify for a particular department at a university institution because your matric marks are too low, there are a number of bridging courses that institutions offer to allow someone into a particular field.
The best idea would be to contact the South African Department of Basic Education, they should be able to provide a list of institutions whereby you can improve or upgrade your matric with more specific details.
*Note: Further Education and Training Colleges (FET) and Technical Vocational Education and Training (TVET)