2 ways:
1. Wireless Access Points in strategic places to cover the building. How many and where you place them depends on wireless site survey results. You will need to run LAN cables between the access points so that needs to be considered.
2. Wireless repeaters in strategic places. Again, how many and where to place them depends on a site survey. This does not need LAN cables between repeaters but you will need more to cover the same area and you will lose connection speed as well.
Get good AC routers now and it should last 10 years unless technology drastically changes.