Generally speaking, 2 ways to do this:
1 – execute a branch detail upon commissioning. You’ll be in one branch as a LT, and then move into another branch basically in the same timeline you get promoted to CPT and go to CCC (around 5 year mark). If you want to try out Infantry or Armor for experience, but then dive into MI or AG for a longer haul, this might be the opinion for you. Doing a branch detail does not exclude you from being able to VTIP after you’ve served awhile in the second branch you enter, but it is possible it would be a bit tricky.

2 – execute a VTIP (see earlier comments). When you are able to execute a VTIP will vary based on the branch you’re wanting to leave, the branch you want to enter, and if that means you can leave as a 1LT, or have to be a KD complete CPT, or something else based on the unique elements of the functional area. Something else to keep an eye on regarding VTIP (in addition to MILPER and HRC’s website) is the In/Out Chart. This chart is a bit tricky to read the first time you see it, but it’s frequently updated and tells you if your current branch is releasing officers, and if your desired future branch is accepting officers (and to what degree – think red/amber/green % of a chance). If you are interested in one of the 2 dozen branches you can rank as you approach commissioning, but have a long term goal of something that you cannot commission into (Army psychologist, functional areas), this might be the option you should learn more about.

Good luck!