Septic sump - cover/rim removal

I have a sump that pumps liquid from my septic tank up to my drainfield. I had to replace the pump today. The pump hangs on a rope that is run through a slot in the top of the sump. This is covered by a "manhole cover", that has a rim that fits over the top of the sump.
Question: to run a new rope through this slot, I need to remove the rim that the cover attaches to. Not sure how these are typically attached - just friction? Do I need to just pry on it on all sides? Any tips?
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