Inside your base, click on the Command Center and select the "Relocate Base" button. This will open this "Global Intel" window.

The drop-down "Sort By" menu lets you filter the search results. Most players filter the search by "Friends" so they can position themselves close to trusted allies.

Then choose one of the friends listed to relocate to their sector, or hit the "Random" button to go to a random sector. 
   
A pop-up message will ask you to confirm, letting you know that all your platoons will be returned to your base. Any bookmarks you have will be reset, and you'll also lose control of all Resource Deposits. If you're sure, hit the "Relocate My Base" button.

There is a chance that the sector you want to move to is full. If this is the case, a pop-up message will tell you so and let you know that you can try to relocate again to another sector.

How often can I relocate?

You can relocate as often as you'd like, and there is no in-game cost for doing so. There are some events that prevent relocation to keep the event fair for all players.