To get a credit limit increase with Citi, go to Citibank's website or app and navigate to "view account summary." Select "Credit Cards" and choose your own. And then you'll have instructions where you can request a credit increase (or a credit decrease, if you prefer).
Something to keep in mind, though: If you're making a credit increase request out of the blue, you'll have to provide some financial information to Citi. There are more details on its website, but essentially, you'll be offering up either your latest individual income tax return, your latest two months' pay slip, or a Form 16 (a certificate that is given to salaried individuals that contains tax information).
On the other hand, if Citi tells you you're eligible for a credit increase, all you have to do is agree to have it increased - and there's no documentation to furnish.