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Do you mean the crack between the frame and the blue wall? As long as the frame is stable, you can fix it by using caulk (it comes in tubes for a few quid that you'll need a sealant gun to apply). I'd suggest getting good flexible, paintable caulk - rather than one of the cheaper ones that doesn't flex too much.
It's a cheap fix and easy to do, but if you've not used caulk or a sealant gun before, I'd watch a youtube video on it first, as you can make a mess with caulk if not careful.