[Nexus 5 Q&A, Help & Troubleshooting] Powered_By_Linux:
Sure. I assume you mean use the phone with a different carrier overseas. Keep in mind, however, that people have had different experiences with this phone. This is just what worked for me. Backup all your data before doing anything as well, just to be safe.

After talking to FreedomPop, they said you have to use a different ROM than the FreedomPop ROM that is preinstalled to use your phone with a different carrier. According to them, you also have to have the FreedomPop ROM installed to use their service, so you can't just install the FreedomPop app. Keep in mind, however, that some people have had success with just installing the FreedomPop apps on a custom ROM and using their service, but it's not recommended by FreedomPop.

You need to deactivate your nexus. Call FreedomPop and deactivate it, but make sure that you don't cancel your account if you want to use FreedomPop when you get back from overseas.

If you don't know how to navigate fastboot mode, learn here: http://www.android.gs/boot-nexus-5-fastboot-mode/. Also, you should turn on USB debugging. Tutorial here: http://www.kingoapp.com/root-tutorials/how-to-enable-usb-debugging-mode-on-android-5-lollipop.htm.

The first thing is to check if your bootloader is unlocked. Boot into fastboot mode. On the bottom line of text, it should either say "lock state - locked" or "locked state - unlocked". If it says unlocked, you're good to go. If it's locked, you have to unlock it. Here's how to do that: https://www.androidpit.com/how-to-unlock-nexus-5-bootloader.

If you DON'T want to return to using FreedomPop after you get back from overseas, you don't need to make any backups of their ROM. You can follow the guide here: https://developers.google.com/android/nexus/images to have a good-as-new, stock nexus just like if you bought it from the Google Store. This WILL wipe all user data, including photos, apps, and more. EVERYTHING is wiped, but that's OK if you don't want to return to FreedomPop. Also, make sure to run the commands as root in the tutorial above.

Also, you could just use a custom ROM indefinitely if you don't want to use FreedomPop when you get back.

However, if you do want to return to FreedomPop, you need to make a backup of their ROM. For ease of use, this method involves using a custom ROM instead of stock Android while you're overseas. Follow the instructions on this site to get the custom recovery TWRP. Use the fastboot method: https://twrp.me/devices/lgnexus5.html.

Next, you need a custom ROM and some Google apps. Feel free to use any custom ROM if you have one in mind. If you don't, I recommend the latest AICP release, based on android 5.1.1: http://dwnld.aicp-rom.com/?device=hammerhead&type=RELEASE. You will need the android 5.1.1 bootloader and modem, so make sure your device is upgraded to 5.1.1. For some reason my nexus couldn't update through the FreedomPop ROM, so you can manually install the 5.1.1 bootloader and modem via fastboot if you need. See here: https://www.androidpit.com/how-to-update-nexus-5-bootloader-and-radio You also need gapps. Get some here: http://fitsnugly.euroskank.com/?rom=banks&device=gapps. Keep in mind these gapps are only compatible with Android 5.1.x.

Put the ROM and gapps zip on your nexus internal storage, somewhere where you can find it. Boot into TWRP and make a backup of your current system by selecting "Backup" and following the onscreen prompts. This will take awhile, depending on how many apps you have. Next, go to "Wipe" and factory reset the phone. This wipes all apps. Next, go to "Install" and install the ROM zip, and then install the gapps zip. Then, reboot your phone and enjoy your custom ROM. The first boot will take awhile. Now you can pop in a local SIM card and enjoy your trip! When you get back, restore the backup you made in the "Restore" option in TWRP, put back in the FreedomPop SIM, and reactivate your phone.

If you get stuck or need any help, feel free to PM me :)