If you have covered your graduation course regularly and honestly then you just need to brush up little bit.

First of all just go through the syllabus thoroughly

Secondly figure out which are the topic common to both jam and your graduation syllabus over which u have command over

And Then that topics which needs to be given extra attention

Go through the previous years paper just to figure out what iitjam actually demands

Don't stuck wholly in just revising the theory, practice as many problems as you can (this is a personal experience)

Don't let yourself satisfied by solving relatively easier problems,go for the conceptual ones.

Practice as much questions as you can.

All the best!!