Now 14 mil mylar is fairly thick, compared to that which I have used from time to time.
The idea of heating to flatten it out is a bit of a concern though, especially if you want to retain any of the heat-shrinking ability of the sheet for later covering or other uses.
If the problem is a bend, then I would suggest running the bent area
gently over the edge of a table, workbench etc under tension to bend the affected area back.
In my experience there is only one method for removing a fold, and that is to throw it away if the affected part will not straighten after shrinking (that is not a problem with 2 and 5mil, folds usually come out
)