Replacing bottom trim on a glass tank
I will assume this is a glass tank. Get one of those retractable razor knives at the hardware or dollar store. The long wider blade ones that are disposable. May want to buy two, their cheap, handy and you might break one.
Extend the blade long enough so you can get a good grip of the handle while the blade is pressed flat against the glass. Be careful the blade may snap if bowed to much. Work the blade between the trim and glass all the way around the sides and bottom. With a little work and patience you should be able to get it off with little resistance.
Using the razor, scape away any excessive silicone off the glass from the old trim. Apply aquarium safe silicone to the trim and reapply. Not too much silicone or you will have a mess. Use masking or electrical tape to keep pressure against the trim till it dries. I like electrical tape because it some what stretches and you can get a little more hold with it. Tape every six inches should be more than enough.
Let dry for 24 hours and DO NOT try to clean up any of the excess silicone that may ooze out between the trim and glass. You will make a huge mess. Wait till it dries and use the razor knife and just carefully cut it away.
Good Luck