Rebuilding and Preventive Maintenance
Westworld Guide
Re-build
Expanding your control room would eventually unlock the Rebuild Room. What this facility does is literally re-build your existing host from ground up. As you successfully do, the host you are trying to re-build will have better stats than before - at the expense of using lower tiered host to perform the re-build (meaning they are scrapped for good). Think of it as sacrificing one of two hosts to make the other one better. Rebuild is an integral part of the game as it mainly open up better performing hosts that can cater higher requirements of guests as you progress in the game.
Additionally, as these hosts typically reach a higher level, they tend to unlock more roles to play - making them efficient in various activities in different locations. When you start the game, you'll see basic hosts barely keeping up with the demand of the guests. However, as you get a hand on Teddy, it would be evident that he can perform the same tasks at a higher satisfaction rating. As you'd know by now, higher satisfaction tends to get your host off the hook. If you see that the chances would fail (usually marked red), don't push through with the interaction as your host would most likely die.
Do not waste efforts on re-building 1-star host too as they would generally be at the low end of the spectrum when it comes to performing for guests. At the very least, use a 2-star host as the main subject to improve in your roster. When you start in Sweetwater, you'll eventually unlock all facilities there and have the option to upgrade them. Even if new locations becomes available to you, you should focus on performing activities in Sweetwater until you get a decent roster of hosts. The reason for this is that Sweetwater has a passive buff increases your rewards by 20%. This means you also get 20% basic host codes from all the activities that you'll do in a day. In average, this equates to roughly 20 basic hosts that can ultimately be used as sacrifice to level up your better line-up of hosts.
Additional Tip: Each of the guests have different affinities that are generally indicated by the color of a hat (see their stats). Before you start your interaction with guests, you might want to pick a host that is of the same color as his/her guest. What this will do is activate the host's cornerstone -- which will give it better interaction with his/her guest, which equates to better rewards upon completion of the interaction.
Preventive Maintenance
Despite having processed a lot of interaction without a hitch, be wary that hosts do get tired every once in a while and will start to malfunction. To check for any risks, you can click on hosts that are currently inactive (no interaction with guests), and find their Glitch Risk. If you see values 5% up, you should take them to Diagnostics as preventive maintenance. This wont take much of your time, and would basically save you a host in the process. A host that malfunctions will generally die - and would take a lot more to get it back online (as it will require you to fetch several items to perform the repairs).
If the actual death of a host happens nonetheless, to fix them, you need to go to the Body Shop room. Apart from having to obtain the required items to fix the host, it also takes quite a bit of time to perform the fix.