I am assuming that you are referring to an extension of a 90 day tourist visa received upon arrival?
You are allowed to stay in Colombia for 180 days per calendar year on a tourist visa.
First go to Immigration Colombia and they will tell you what you need; basically a photo, if you have not had a recent visa other than a tourist visa. They may ask you for a flight confirmation to show that you are leaving the country within the 180 days, and you will have to pay a fee of about COP160k. This can be paid in cash in advance at the bank they will advise you, or with a debit/ credit card at the time of processing. The visa will be extended from the last day of validity of the current visa for a further 90 days. It takes about an hour or so. They are quite friendly and it's not a hassle.
However, do not try and stay more than 180 days in a calandar year, you will be deported and maybe fined and they may place a restriction upon returning.
Hope this helps!