How to remove markings without erasing whole thing

Hello! I am new to MuseScore. I have been writing a piece (without MIDI) and, to be honest, it has been frustrating and time-consuming. I'm slowly figuring out some cheats. I can't seem to figure out how to do certain things, such as:
How do I remove a tempo from the beginning without erasing everything I've done since writing it in?
Similarly, how do I change other markings without deleting the entire piece? When I go to the position and click the same marking on the palette (like you would to remove a note) it just adds more markings.
How do you create a large slur over an entire system?
Is there a way to change all notes in a chord from one length to another? Such as all from quarter notes to whole notes in one swoop? Or do you have to delete them all and add them again as whole notes?
Do you have to create an entirely new score to add lyrics to a piano accompaniment?
Thanks!