If you haven't gotten it yet. Take a look here and it might be a little easier to understand what you need to do.
First you can squirt a little liquid wrench or similar with the little sprayer tube right on back side to help it some even though you stated they just put it on. But if it was corroded/rusty and little then it's sorta like cross threading a brand new nut. Won't move easily.
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Now here should be what your seeing. your arm with lever pushed in meaning locked.
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Little flat piece on bottom that has curled up edge. Reach up with finger or use flat screw driver and pull it down like this.
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Now it's unlocked. These 2 pics shows locked on top and unlocked bottom and show you what your working with but can't see yet. See how it just slides in to groove on arm to prevent arm from coming off.
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Of course yours is still on so now unlock yours by pulling it down.
Next grab you a flat screwdriver and hold it in a fist with like your knuckles against the glass and put tip edge only maybe a 1/16" under edge. Just giving you some leverage to pry it up easier and grab arm on left side same way fist like. Now just even pressure with a little wiggling side to side to pull it off. Screwdriver helps with holding it level so it doesn't bind on shaft trying to come off. And this will be what you should.
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Remember to set the clock position of blade before you snap it back on, meaning where you want to blade to stop at when it is off. Oh and put some lube on it before reinstalling it. Hope this helps, but even of you already got it off just say it helped. That way the typing and pics will have seemed useful.