Trying to add minutes to HH:MM to 00:00 in Char(5) Field

I'm pretty new to SQL, someone has asked me for help and I'm trying to assist; I've installed SQL 2005 because this is the version they are running on.

My task:

They have a table called 'appTime' which is a char(5) type contains many times such as: 12:12 08:15 6:22 etc.

I have to increase the minutes in random increments between 1 minute to 15 minutes, I have no idea how to do it.

I've tried to just randomly move the same number to another field eg: UPDATE SET newTime = AppTime and failed..

I'm not sure how to do it, I'm not sure why the time is even in a char type but I assume their software required it.

Any tips, help, assistance is GREATLY appreciated.

Thank you