Redmine

Hi, I need advice.
Length comments are limited to 255 characters. I removed the obligation to control the number of characters in the source code. Now when saving a longer comment omit error, but it saves only the first 255 characters. I would remove the restriction directly in the database time_entries - comments - varchar (255), but I do not know how to do it. I want to change the type varchar to text blob. Can you help me, please? My English is bad