Can I upload by an original st or do I have to dawload from cat website I am confused in.

For caste certificates follow the pattern given on the CAT websites. They have explicitly mention to follow that format and accept the only format they have mentioned on the website.

If you require more time to make your certificate according to CAT website format then you can upload your original certificate and you will be given relaxation in time to make up your certificate till your college interview time.

Before your any college interview every college goes through your all documents exclusively and you can't get any benefits with any document which is not appropriate according to their demand.

Anyway that is a latter process which starts in Feb so you will have enough time to renew your certificate in any case if you are not able to do now.

So in short go with the format mentioned on the CAT website.