In order to add something, you have to remove something first. In this particular example you'd have to patch over all the text from the original movie, which in itself is a major piece of work. Lots of masking, tracking, using content aware fill and clone stamps, duplicating layers and so on. Once you have clean plates, it's easy enough to add new text on top. Anyway, the opening sequence of this particular movie is quite long, so you should start by looking for simpler examples to practice and even if you do, it's going to take time. There are no magic buttons. Start by reading the online help:
After Effects User Guide
Mylenium
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