To measure the wear on your city bike's tyres and to know when they need replacing, you need to inspect the tread.
Usual signs of wear include:
- tread starting to fray
- the appearance of a series of micro cracks
- low tread depth
- the carcass becomes visible.

In all these cases the tyre is worn and needs replacing.

To remove your tyre, you need to deflate the inner tube until no more air comes out.
Once deflated, use a tyre lever to remove the tyre from the rim.

To put the tyre back on, place one side of the tyre into the bottom of the rim. Next, inflate your inner tube to approximately 30% before putting it into the tyre (inflating it beforehand reduces the risk of pinching the inner tube).
Then, place the other side of the tyre into the rim, starting on the side opposite the valve.
Before inflating the inner tube all the way, check to make sure it isn't pinched between the tyre and the rim.