Rocker arm (example part# 11337530979) or intermediate lever (example part# 11378645501)?

If it's the first, I would guess super easy if you have the right tool to compress the spring valve spring. Probably nearly impossible if you don't since you can't get it back on without releasing the tension. You can rent the AGA set for ~$200, maybe get a break from someone if they know you're not going to replace the seals an only need it for 10 minutes worth of work.

If it's the lever, you just need something to pry the retainer spring up enough to slip it back on and some gentle/careful hands.

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2007 X5 4.8i - 12/06 Build - Silver/Black - 3rd Row