my iphone 5 has a weak signal need help

Try to reset the settings.

Settings - General - Reset - Reset Network Settings

Try to reset your device. This won't delete your data stored on the phone.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

https://support.apple.com/en-us/HT201559

Do you have problems with signal at home or anywhere else outside the office?

If you have problems with signal in your office, this may be connected with its location, the location of closest carrier base station etc.

There's not a whole lot you can do about it. Some building materials tend to block cellular signals. It's a fairly common problem.