You can contact them via email and need to provide the necessary documents.
I just submitted mine in KLCC and the staff helped me to fill in a form.

That's all for me.

last time i did ask Maybank via FB messenger. here is they answer

"you may email your request to mbbcardservices@maybank.com.my together with your latest 3 months payslip and EA form.

Generally, the processing time for conversion to upgrade the credit card will take 10 days (excluding Saturday, Sunday and Public Holiday) upon receiving by our processing department. For new Maybank 2 Cards Platinum Cardmembers, finance charge of 18% p.a will be applied. The applicable tiered finance charges will only be effective until such time when you have the minimum 12 months repayment record with us."

just that i dont like the bold wording part. where the 18%pa will be applied. now i get 8% since i got good record for 12months.

if i upgrade, i need to make sure i pay promptly everymonth to avoid that 18%pa. hahha

btw, got any diff between gold and platinum  except for the complimentary insurance ?

To add on, I submitted three months latest payslip. That was back in early 2015, I believe still same process.

yes, they hv same interest. but hv diff tier

something like this:
Tier 1: 8.88% (for prompt repayment of 12/12 months)
Tier 2: 17% (for prompt repayment of 10/12 months)
Tier 3: 18% (for prompt repayment of less than 10/12 months & New Cardmembers)
please refer back to here
http://www.maybank2u.com.my/WebBank/M2C-Plat_PDS.pdf

so mean, ur will be reset back the tier 3 once u upgraded to platinum, this wat the Maybank FB told me.