Before you upgrade you need to look at what is wrong with your computer are you running out of space? Are you finding your computer too slow? Does it not look as nice? 

If you are running out of space you may need to look at getting an external hard drive or buying an internal depending on whether you have space for one.

If you are finding you computer too slow you may need to look at getting a new processor or graphics card

Or if you find it does not look as nice you may need to look at getting a new monitor with better quality.