You'll have to remove the muzzle device (if it has one), remove handguard, remove gas block, remove barrel nut, and remove barrel. Install new barrel in reverse order. There are tools you'll need to make this all easier. Upper vice block and barrel nut wrench are the main ones. 308 and 243 share the same parent case so you can re-use your BCG and magazine. If you've never done any AR based mechanical work, there is plenty videos on the webernet. Let us know how it goes.
For me, I get more pleasure from building them than shooting them. You might find it more rewarding to start from scratch. Aero Precision M5 upper/lower and go from there. Nothing wrong with DPMS, but it's something to consider.
And welcome from Cajun country!