How could I fixs my phone battery becuase it dosent charge fully 3

I think we might need to know how old is the phone? About how many charge cycles has that battery been through? Also I would ask have you used a charger that was not what is recommended for that phone at all?

If the battery has a lot of charge cycles, it may be weak and not fully charge or hold a charge as long as it should. At which case you would need a new replacement.

Looking back, has the phone been plugged in to a outlet that might be questionable or a charger that might be bad? Those conditions can cause a battery to fail.

You can try to separate the battery cover and inspect the battery to see if it is swollen or shows other signs of damage. Though it sounds like a weak battery that needs replacement.