Change, alter, and modify are all pretty much the same. Of the three, "change" is probably the most common. "Shift" has a similar meaning, but is used when talking about a movement of location. "Let's shift the couch back just a bit." "The tectonic plates shifted causing an earthquake." "Convert" is more specifically turning something into something else. "She's trying to convert me from Catholicism to Mormonism." "Convert this file into a .jpg." And "mend" means to fix. "The doctor mends a broken arm." "Mend your behavior if you want to go to the party."