Open Control Center and drag the brightness slider up
Go to Settings
Tap Display & Brightness
Move the Brightness slider to the right
Turn off Auto-Brightness if it is limiting screen brightness
Go to Settings
Tap Accessibility
Tap Display & Text Size
Turn off Auto-Brightness
Turn off Low Power Mode if enabled
Go to Settings
Tap Battery
Turn off Low Power Mode
Disable Night Shift if it is making the screen look dimmer
Go to Settings
Tap Display & Brightness
Tap Night Shift
Turn off Scheduled
Turn off True Tone if needed
Go to Settings
Tap Display & Brightness
Turn off True Tone
Clean the screen and remove any dark screen protector if needed
Restart the iPhone if the screen still seems dim
Check for overheating, since iPhone may dim the display automatically
Update iOS if the brightness issue continues