How To Change Time Signature In Altered bar

I am a beginner to musescore so like everyone else trying to work my way through a few scenarios.
So I open a new score 4/4 add then add four crotchet notes.
Thats it just four notes
I then delete the very first crotchet.
I wish to make the very first beat a crotchet rest,no can do.
I dont know how to make this first beat show up as a crotchet.
Now I appear to be left with 3 crotchet beats in the bar.
It would appear as if the software has turned the bar from 4/4 into 3/4
Quite funny I cant turn the bar back into 4/4 timing
Can anyone tell me how to turn the bar back into 4/4.
Thanks
Gus