I've bought a second hand bike recently but the back tyre is bald but I can't find the correct size tyre for the back wheel, I've tried everywhere, so my question is: Can I fit a 1.95 in tyre on my 1.90 size wheel?
1.9" = 48.3mm 1.95" = 49.5mm
If you think your frame can accommodate an extra 1.2mm tyre width (that's an extra 0.6mm each side) than you're safe. You may well be able to go bigger if you wish. You should allow 3-4mm minimum either side of the tyre in most scenarios but many performance bikes will use tighter clearances.
Modern mountain bike sizes with decimal widths use standard rim sizes and are compatible. A 28x1.95 tire will fit fine on a wheel that is currently home to a 28x1.90 tire for example.
This is NOT true of tires using the older fractional system. You cannot replace a tire marked "1 3/4" with one marked "1.75".
(Also note do not get a 27.5 thinking you'll stretch it onto a 28, they do not stretch. Sorry if that seems pedantic, people have tried!)