My PC (OS Ubuntu 21.04) freezes daily.

Thanks to Ask-Ubuntu forum, my PS RAM was increased from 2Gb to 4Gb. But freezing remains.
I searched programs (installed by default) and uninstalled all I do not use.
The question is how to increase RAM, say to 8Gb?

Test = = total = used = = free = = shared = buff/cache = available

Mem = = 3.5Gi = 2.2Gi = 613Mi = 350Mi = = 721Mi = = 768Mi

Swap = = 5.8Gi = 322Mi = 5.5Gi

Thank you.