It wouldn't be too difficult in the sense of it being complicated but it would be time consuming.
I would suggest that the best approach would be to remove the window completely and then spend some time cleaning off the remaining silicone. This takes a fair bit of time - although there are some products which are designed to help with silicone removal. I've never used any of them so do your research and test that it won't affect your paintwork.
Once the silicone has been removed from the van and from the window then I would check if the edge of the metal where it was cut has been painted over - if not then paint it to prevent rust forming.
To refit the window I would suggest using a two part vehicle window bonding kit.
Clean and degrease the van and the window to ensure a good bond.
There is a primer which you apply to both the vehicle and to the window which dries in 10 mins and then a sealant which you apply to the window and push into place.
I used a cheap caulking gun for this and it was a disaster as it simply didn't have the power to push the sealant as it's thicker than the usual sika flex and bathroom sealants I've used in the past. It's worth spending $20-30 on a good gun to do this task IMO.
Once pushed into place you can use some gaffer tape to hold the window to stop it slipping as the sealant cures.
There's a video which I found helpful as it was the same window manufacturer I used, but the basic principles should apply to yours as well.