Hello Mohammed,

Sand should only be used to level lawns when the substrate soil is sandy as well. https://hgic.clemson.edu/factsheet/topdressing-a-home-lawn/

If you are on a sandy loam soil, then levelling with a mixture of topsoil and sand is appropriate. In general, the topping material should be the same as the substrate soil.

There are three ways that are effective in levelling a bumpy lawn:

If you have large, low areas, you can peel the grass back or cut it out in uniform thickness strips. You can then fill in the low area to within the strips’ thickness and replace the strips of grass you cut.

If low areas are smaller or very numerous, you can use soil similar to your substrate and add thin layers over the low area every week of two. The objective is never to completely cover the grass plants. This will suffocate and kill the crown. However, if you continue to add thin layers of soil, allowing the crown to reestablish to the new height each time, you can eventually fill in the low areas: it is just a slower process. This is similar to topdressing. For a substrate other than sand, topsoil or compost is an acceptable topdressing medium. Topdressing is generally most effective when it is accompanied by core aerating. Now is an acceptable time to start topdressing, but realize that if you have deep low areas, topdressing ay take the rest of this growing season and into next season to be effective.

Finally, you can completely fill in all the low areas with soil similar to your substrate and then reseed the filled-in spots. Completely covering the crowns will smother them, so the grass will not reestablish higher in the soil, so you will need to reseed the areas.

Here are some references on top dressing

https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3710.pdf

https://nwdistrict.ifas.ufl.edu/hort/2016/03/11/why-do-people-apply-sand-over-their-lawns/

If your lawn is just bumpy and does not have localized low spots, rolling may be a solution. Effective rolling of bumpy lawns is addressed in this article:

https://yardandgarden.extension.iastate.edu/how-to/bumpy-rough-lawns

By late September, you may want to cease the topdressing regimen and core aerate, top dress with compost and overseed. Then continue the topdressing to level the lawn next spring.

Hope this helps.