Tap on the "+" icon at the bottom of the screen. Select the images you want to include in your video and tap on the "Next" button. TikTok will automatically add transitions between each shot. You can then add music and effects to your video.

Unfortunately, the answer is no. Instagram doesn't currently have a feature that allows you to edit the video itself of a Reel after it's been posted. However, you can still change your reel, such as changing the caption, resizing the cover image, and adjusting who's tagged.