My wife bike is using Shimano ST-EF50 shifters.
She started to complain that the left side, the thumb shifter she needs to press it all the way in instead of half the way in order to change gears and it never used to be like that before.

Any adjustment I can do in order to change the thumb shift lever travel?

https://www.parktool.com/blog/repair-...ur-adjustments

You might also want to flush the shifter with WD-40

Worst case, a new shifter isn't too bad-
https://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1

The 4 finger version are compatible with V brakes or cantis by moving a little plastic plug.

What do you think is the problem there?
I heard that there is a screw at the bottom of the shifter to adjust but I'm not sure what it does.

Either your DER cable slipped/stretched/??? or the shifter is gummed up or toast.
When on the smallest ring is the cable just snug or flopping around?
Is it shifting to both rings or just one.
With the limited info you gave, what do you expect?

So I'm guessing she can only get to 2/3 rings?

Help us out here.

Sorry guys never did this before, I will try to take the shifter apart in the next few days and see.
The shifter changes from 3 - 2 -1 and reverse but she needs to push the lever all the way in.
Also sometime the chain doesn't change.
I tried adjusting the cable tightness, it helped but not a lot.

I took everything apart and couldn't find anything wrong, looks like everything works like it should, also I found a similar shifter someplace else and it behaved the same.
I have another question about my wife's bike and in general.
I adjusted the front gears, everything moves smooth not issues at all when riding on flat ground.
My wife says that while she is riding up hill many times the bike will not change from 3 to 2, the chain will not go down to the middle wheel.
Any ideas what it can be.

Worn out chain or chain rings or both.......depending on the bike; component quality and age; it may all be worn out. You need to take/post many good pics and supply solid information about the bike and also about what you have done so far. Not trying to be snippy; but your info is too vague for anyone to help you.....

The comment about riding up hill and issues with shifting off the big ring sound like she's not soft pedalling during the shift. Andy.