yes its possible to mess up but fairly unlikely even for the beginner.
but after watching a couple of youtube vids even the most inexperienced user can change his thermal paste.
it requires the removal of 1 fan header and depending on the cpu cooler used. either a thumb or screwdriver to remove the heatsync locking mechanism
you wont be removing the cpu so theres next to no chance of damaging that due to mishandling.
and the cooler removal itself is just a simple unclip or unscrew the corners and lift off.
clean with the appropriate amount of tim, re-apply a pea size amount to the center of the cpu, put the cooler back on top and lock it down.
connect the fan header and job done... it should take less than 10 mins total. (even if you havent done it before)
so no mate i wouldnt recommend you take it to bestbuy, they will overcharge you for something that is very simple and straight forward to do.
https://youtu.be/AyKPsS8GX3I?t=4m50s
intels own guide.